Heating Bed with external Controller

A friend asked me to help him out with adding a heating bed to his da Vinci mini W+.

Here is what I came up with:

Parts needed:
- Heating bed incl. cable
- Ramps 1.4 (or better)
- 20A power supply
- USB stepdown converter
- 40mm fan
- some wood, screws and the usual (soldering station, cable...)

All my designs are in this thing, but you will need this too: http://www.thingiverse.com/thing:2228053

Please see the marked up photos for details.

If you want to modify the design - feel free, it's SCAD. Necessary modules are on github (write) or in my things.

And yes, the casing is quite big. The goal was to fit it under the existing printer.