This will fit the home depot chrome towel bars and 5/8″ wood square stock. The opening is 16mm. The mounting holes are for bell type motors with three screws (that’s a DT850 in the picture) but I plan to upload a version for standard four screw motors. It’s designed so that the wires can go inside the tube without having to wrap around the end and be exposed to crashes. I have flown with these more than a dozen times in the last year and they perform great.

I have included the source files so if anyone wants to modify it to allow for different motor mounts, please do and let me know so I can upload it or link to it from here.
Update: Thanks to wolfend for his mod for regular 16×19 motor mount: http://www.thingiverse.com/thing:799007

Check out my yaw control mechanism: http://www.thingiverse.com/thing:737588
and landing gear: http://www.thingiverse.com/thing:737647

I usually print this in ABS, 2 shells, 50% infill, .2 layer height. Vary the infill if you need it stronger or weaker.

Print this with the long flat side on the bed with support. I have tried printing it vertical without support but it becomes too weak along the print layers. The orientation of the part on the bed will affect the way support is generated and how difficult it will be to remove.