XYZDesignPro posted about this design on the forum but didn't know how to upload STL files so he asked me to do it here.
This is a modification (remix) from this part here by selmo:
http://www.thingiverse.com/thing:767741
The hinge pivots needed to be modified to suit the narrower front bezel of the UM3 vs the UM2 for which it was originally designed
More information on this topic on UM forum:
https://ultimaker.com/en/community/39764-um3-door
Technical
From XYZDesignPro: I used .090 Lexan for the door. Available at Home Depot pre-cut to 12" X 18" size. The door is 12" X 15½". Just cut 2½" from one end and you're good to go. Screws go directly into the Lexan. They will tap their own threads with a good power screw driver, if just the right hole is drilled, eliminating the need for nuts. Best to glue the handle in place.