BCN3D+ or MendelMax double spool holder

From the original single spool holder designed for [BCN3D+](http://www.bcn3dtechnologies.com/) by BCN3D Technologies, I would like to share with you this "derivate" in order to upgrade it into a **double spool holder** which uses less parts than two single holders. It's great if you have double extruders or if you just want to have two filament spools ready to be used.
The holder lays/clips to a T-slot profile (20x20mm).

If you are **upgrading from a single holder** you will need:
- print