i've seen the kits. very definitely pretty. also very definitely pricey. i have always wanted to make some so have also wondered whether or not i'd save piles of money if i put together my own kit. you already have to buy the foam, which means you'd have to mark and cut the patches yourself. that means you need to buy the cutting tool(s) too, i'll bet.
sooooo i'm thinkin' the supplies list for a DIY might go something like this:
foam
ruler(s) - which we already have
at least one exacto knife
templates for the block pieces- which we either have or could easily make
a nice frame with at least a 1/4" inset to "nest" the foam pieces
scraps - which we all have already, too
glue - which we usually have on hand
you could preposition the foam pieces and use some quarters as spacers so there's be a place into which you can stuff the pieces of fabric. a credit card would probably work nicely for pushing the edges of the fab pieces down into the gaps.
i'll bet you could also just wrap the foam pieces before you position them inside the frame. it would all go together like any other puzzle.
i think it would cost less AND you wouldn't go to all that effort to make the same block(s) from the same fabrics as the other hundreds of people who also bought the commercial kit(s).
just my two cents.