I bought this set of four interlocking floor mats for <$10 at Harbor Freight. they're about 10mm thick. they're just the right size to be cut in half and trimmed down to fit in the drawers and I'll have a few extras.
Here's the SAE drawer. I drew one line with a yard stick along the bottom and started laying out the sockets with even spacing. it's not perfectly square, but it's close enough. I mapped them out on the "back" of the mat so it will look clean when I flip it over. I have some space left over, which might hold some hex keys or something:
I've cut out about half of the the socket pockets. this took some trial and error, but I found this to work:
put the box that the toolbox came in on a table top. it's four layers of cardboard so you can jab straight through and not worry about dulling the knife on the tabletop or scratching up the surface.
extend utility knife blade all the way out (about 1") and start jabbing. cutting out each square takes a steady hand, but you can kind of saw through it to keep from sliding and slicing too far.
flip the mat over and carefully cut out the rest of each square nice and clean.
^this socket fits perfectly in this spot.
when I flip the mat over, it looks like this:
bonus tip: for the very small sockets, cut the hole out about 150% of the length of the socket. they way, you can fit your finger behind the end of the socket to pry it out. you don't want the fit to be too tight so that the sockets fall in and are a challenge to dig out of the pocket.