I figured out how to make this VERY modular so you can add Zeros as they become available, and only use as much filament as you need.

DOVETAIL JOINTS! Or maybe even a tongue and groove joint...

I drew this up at work just now...

Print 2 ends and 2 rails to enclose 1 Pi Zero. Flip a rail and you have the bottom! Thereafter, to expand just print 2 rails. And match the keys. You may give up a bit of smallness in the design, but this makes it so you can add ONLY as many as you need. Still, I'm going to try and design a minimal footprint per leaf. Maybe 15mm. If that leaves me with not enough space for the Zero itself, I may have to just make a combined rail-leaf. Really it would be a leaf with dovetails... I want to avoid using supports for the prints.

The leaves will be much more important to the strength of the case, but that could be fixed by using a ring around the ends. I design this to use minimal supports during a print