50g Mini Spool Remix

This is a quick remix of [johannesbehr's design.](https://www.thingiverse.com/thing:129173)
I mostly reproduced the geometry of his design in OpenSCAD so that I could adjust some dimensions and clearances slightly. The walls weren't thick enough for my taste/printer.
*Changes 5/24*
I've replaced the "Case" piece with the simpler "Guard". The Guard modification idea came from another [remix](https://www.thingiverse.com/thing:998090).

This holds 50g filament like often comes in sample packs/