Or so they say...
From what I've seen these third party inserts actually push up and increase the overall size of the box. They're also very expensive. Some have argued these are the ideal solution, because it takes time to cut the holes in the foam yourself (camera foam isn't so cheap, especially when it comes in a big plastic/metal case, and it wasn't tailor made for your game although "pick foam" lets you rip out the blocks you don't need to form the "void" into which your pieces will go, as it's pre-perforated). Plus they like the idea that when you open the box and show it to your friends, they will be impressed with how straight and perfect everything looks. To those all I can say is I guess that's an admirable goal, but for me, I like to buy stuff for my hobby, but I also like to save money when a solution presents itself, even it might require me to do something more than click an order button and open a piece of shrink wrap (I become part of the process of creation). Each to their own right?
You could of course just buy an actual pluck foam block from feldherr as in this example ($24.99).
So for the more DIY types... the whole "foam" thing is something I've been doing myself for years, but I never undertook a 100% replacement for the "battle foam for mythic tier" thing. Let's use a recent post from our own FainFlynn as a general guideline to get us thinking. Feldherr foam is on sale for € 49.99 (approximately $56.55 USD). Which is about HALF the price of the much ballyhooed "Battle foam". You don't need memory foam, this is just to put a little soft thing between your minis and the box. I guess you could use cotton balls (although I foresee those getting snagged on sharp bits and making a bit of a mess each time) or put them in ziplock bags and fill the gaps with packing peanuts.
The foam we're talking about is squishy, rather than the super cheap stuff that makes up packing peanuts in shipping boxes, or the harder foam that is used for camping sleep pads, or the stiff "craft foam" (the term is so generic) that is like a hard rice crispy treat (like those big foam balls you can use to create sculptures). This is the same kind of stuff I'm talking about you find inside camera suitcases, or used as pads (not the expensive "memory foam") for beds or chair seats. It's firm but you can rip it apart in your hands pretty easily.
Back to the "foam" that's being sold through these 3rd party gaming shops:
It's gray (foam comes in many colors, the stuff I use is white and turns yellow when exposed to air and sunlight, but you can also get it in green and other colors). If you don't care about the color, first of all, there are some other options.
Let's look at the size dimensions. You'll be cutting it (I use a pair of long sturdy scissors, like those used for dress making, but also an x-acto knife, that's a hobby knife with replacement razor blades). You may need this anyway if they screw up at the factory. I like my foam to be snug rather than just a void into which I throw my miniature. You can sculpt to your heart's content, just don't cut yourself. Although this person got some results with an electric kitchen knife (or here), or even a bread knife. Videos abound showing people cutting the foam even better than I've done it, with tools they have at home.
HB060A001: 394 mm x 294 mm x 60 mm total height (50 mm usable height + 10 mm foam base)
HBMFFQ040BO / HB040A001: 394 mm x 294 mm x 40 mm total height (30 mm usable height + 10 mm foam base)
HBMFFR045BO: 394 mm x 294 mm x 45 mm total height (35 mm usable height + 10 mm foam base)
The above converted to inches is 15.51 x 11.57 x 2.36; 15.51 x 11.57 x 1.57 and 15.51 x 11.57 x 2.36 inches respectively. That's if you want to throw away the existing trays and replace them with squishy foam ones that size.
What you can be buying instead are various furniture foams (also known as craft foams, furniture pads, chair cushion, etc). It comes in high density (probably more than you'd need and tougher to cut) and regular density. It comes in big blocks or sheets and you cut to size.
Examples (just looked online, you may find better deals in-store):
(walmart. a search for furniture foam reveals many results...)
https://www.walmart.com/ip/Morning-Glory-22-x-22-x-2-Regular-Density-Foam-Cushion/44106465
Morning Glory high density 18 x 18 x 2 craft foam cushion $9.50
4-Pack White Polyurethane Foam Cushion Inserts; Square 18x18x1 $28.99
Walmart and amazon aren't your only choices. There's also Joann Fabrics which has many types of the right size, and maybe your local fabric/craft/bedroom shop. Or this place will cut to size. Furniture/upholstery foam is very common once you start looking.
Actually since I've done business with certain companies like MPC, they include sheets of foam in their package which I have repurposed as cushioning for my flambeau plastic tackle boxes that I pack my own painted minis into. (I also use their "lure" boxes to store/display 6 inch action figures, but that's another story).
The other thing to consider is despite what we've all seen, the 2021 Hasbro Remake miniatures are NOT as fragile as the classic 1989-1992 Citadel plastic minis (or resin 3d printed miniatures which are the most fragile of all). Nor are they as fragile as traditional metal (ie: Dark Heaven reaper) miniatures (that can bang against each other during travel and bend and break off glued pieces). The plastic has enough give that it won't break where some other figures would and they bend back into place often with just the fingers and don't leave stress fractures in their place where the old plastic would have.
I would compare the 2021 Remake figures to Reaper Bones (or reaper bones black) in terms of their stiffness and overall feel. Despite some having tiny or thin parts, they're not fragile at all.
Yes, some Remake miniatures did arrive broken for people, but there we are talking about factory/production defects, not damage from being in the package or from play (ie: improperly glued pieces mostly). So whether you actually "need" something like this is up to you. Game shops are always selling you that one more thing you need. You need the dice rolling box (what, no dice tower?), the dice rolling cup, the dice bag, the card protectors, the deck protector box to protect the cards in sleeves, the card trays, the plastic mylar bags to hold your booklets, trays and bags for your tiles, the box protectors and the totes to hold the boxes. What you decide is needed is up to you, in the end (see my signature for my personal solutions to some of those).
If you're like me, a pre-made product like this would never be enough, because I'm always adding little things to the box, and so there is always going to need to be another box or another spot in the tray, etc. This is why I use the tackle boxes, with a little (very thin) foam on the floor of the box and cut little square spacers of the furniture foam that I can shift around as needed, and a thin sheet to tuck those monsters in at night. Use what works for your in your budget. For me, every dollar I save on these little accessories is another dollar I could put towards another mini or bottle of paint (acrylics for miniatures can be obtained pretty cheaply too as I discuss in another link in my signature).
This post contains many links and pictures but it isn't meant as an endorsement of any products just sharing my thoughts on availability of materials for solutions