Warning: The cover is as of yet untested. Tell me in the comments if it needs adjusted.
This is a drop-in replacement for the MP Select Mini's extruder arm and cover. After mine broke the day after my printer was delivered, I found Makerbot's Replicator 2 extruder design to be the same except for the bearing size. I modified it in Onshape, and the bearing from the MP Select Mini fits snugly. If you had to replace parts other than the arm or cover of the extruder, Makerbot's other parts should fit fine. I can't find any dimensional differences between my part and the original, but your experience may vary. Tell me if it works in the comments!
Someone reported some model issues with Simplify3d. Cura and Slic3r don't seem to have any issues with it but if you do end up with problems, download the "fixed" version and try that. I also added a file with both the cap and the arm.
View in Onshape: Arm|Cover
(Also available in .step)
The stronger the better. I recommend 3-4 shells (1.2mm for some slicers), 0.2-0.3mm layer height, and high infill.