I found on Green Stuff World's website what I think you are looking for, an item they call Rubber Steel Sheet. It is dark gray, almost black, and it sparkles. It comes in single 11-3/4" by 8-1/4" sheets, is self-adhesive, with reference number 436554360475. It is priced $4.31 USDUS. It can be cut with a hobby knife, but it takes a lot of heavy strokes. I'd recommend metal shears. A sheet fits almost perfectly in the bottom of a Sterilite #1963 and #1965 latched box. In an 8.1L Really Useful Box there is a gap of about 1/2" on each long side, with a negligible gap on the ends.
GSW also sells a same-sized Self-Adhesive Magnetic Foil Sticker which is just a rubberized magnetic sheet, not as strong as the steel sheet, and not suitable for the bottom of a box. Its reference number is 436554360468. (Make sure you use the proper number. I was careless once, and am still working my way through the 10 rubber sheets I ordered by mistake.) However, they are strong enough to use under your miniature's bases and can easily be cut with a hobby knife or heavy-duty scissors.
While you can cut the magnetic sheet to fit your bases, you've probably noticed by now that GSW also sells pre-punched stickers for many of the popular base sizes. I use them, and like them. I will point out the 25mm sticker is slightly proud of GW's bases (nothing a little texture paint won't fill), and the 40mm base is a little shy (precise placement required with the hollow bases), but the 32mm stickers win the Goldilocks Award for being just right.
I find GSW's pricing to be more than fair, and even with shipping to the US from Spain, their products are a bargain. Shipping time are fast; I just received my latest order as I've been typing this, which I placed about a week and a half ago.
Don, the HobbyDr
experiments with using recessed bases for figures with cast-on bases.
