I've been making keycaps out of polymer clay. I bought two sets of blank PBT keycaps for the #yet another dactyl build in white and black, and after reserving a checkerboard's worth from the appropriate rows, I still have several left over.
I didn't know much about doing this when I started, but I will tell you that ABS keycaps will melt in the oven, so use either PBT keycaps or air-dry clay.
My daughter would get a kick out of these -- she often sculpts little things like this with polymer clay just for fun. Your detail level is amazing here!