Okay, printed version 4 yesterday with 14mm x 14mm switch holes. The switches snap in satisfyingly, but unseat a bit when pulling keycaps. I might hot glue them in place. Superglue would be better, but more likely to seep in and ruin the switches.
For version 2, I pulled the thumb switch closer, made the switch holes 15mm, and minkowski'd out the sharp corners. The switches don't fall through now, but there is a gap that's too big to fix with hot glue.
Have you considered chording to make it do even more with the same number of keys? Chording is something alien to me but it strikes me as the charlieplexing of the input world.