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.