Duet, DuetWifi & DueX4 mount for 2020 extrusion

A simple mount for the Duet., DuetWifi and DueX4 to 2020 AL extrusion.

The SCAD file uses cubeX from http://www.thingiverse.com/thing:112008

NOTE: DueX4 mount not tested since I don't have one, so the board mounting holes could incorrectly positioned.

7/8/16 - Added a version that puts the board over the extrusion it mounts on.
7/31/16 - Added a port cover for the Duet 0.8.5, it's in the v0.2 scad file.
The v0.2 scad file also has one for the DuetWifi, but the wifi slot