Flipper Zero ProtoBoard case 2

This is another simple case for the Flipper prototyping board ("ProtoBoard") with 15mm of clearance on the "top" and 3mm of clearance on the "bottom." This case contains more of the ProtoBoard than [my previous case/cover](https://www.thingiverse.com/thing:5411771).

I needed a simple, mostly-closed cover for my prototype CO2 monitor module for the Flipper Zero.

The case comes together with M3 heat-set inserts and 20mm long M3 bolts.