Bracero: The Filament Side Guide for Prusa Mk3

Print the main body on its side. 

I use flanged bearings to ensure that the rings don't slip out.

Found here: https://www.mcmaster.com/57155K629

Flanged Ball Bearing, Steel, Shielded, Number 608-2Z

Snaps into the main frame of the printer.

Originally made to satisfy using filament spools that cannot be mounted to anything like walls and such.