This is a rear panel to hold the back of a Duet along with a 40mm electronics cooling fan. See http://www.thingiverse.com/thing:965396 for the matching front panel. It can accommodate a 40mm fan for electronics cooling, although Duets don't generally need cooling unless the motor currents are 2A or more.

Update 2019-06-19: one of the feet broke off my original rear mount, so I have modified the design to have greater strength. The updated version needs two M4x20mm screws instead of the original M4x16.