MultiPi -- Raspberry Pi Cluster Case - With GPIO slot

Remix from nice https://www.thingiverse.com/thing:1573414

- Added side GPIO slot.
- Increased SDcard, microUSB and HDMI slots size.

I struggled a bit with Tinkercad, not my favorite tool. As jesse76 published his source, I modified its files: https://www.tinkercad.com/things/gzbrEFr6V2c

Same print settings as original.