IMPORTANT NOTE: I recommend printing this mount with a heat-tolerant filament, such as ABS or PETG. If printed with PLA, the current model may slowly deform due to the heat underneath the Ubiquiti UniFi AP-AC, and it could fail after a few days. I am currently (January 28, 2019) testing a model with vents to help release that heat and to prevent the PLA from slowly melting. I will post the new model once I'm convinced it will work for PLA.
This is a replacement ceiling mounting plate for the Ubiquiti UniFi AP-AC Lite and the Ubiquiti UniFi AP-AC LR. (Tested on the Lite model only, but the LR supposedly uses the same mounting plate.) Unfortunately a mounting plate was lost during some renovations, so I decided this was a good opportunity to learn some Fusion 360. :-)
This mounting plate is close to the original in dimensions, with the notches are a bit wider to make it easier to get the AP into place. The locking bar has a very prominent slope on it to help lock it in place easily. There are two versions: a thinner one and a thicker one.
This model may lack the carefully engineered plastic-saving ridges of the original, but it makes up for that with a low infill percentage.
The main thing is that it does the job. :-)
Depending on feedback, I might change the locking bar. In the original it's actually offset "upwards" a bit from the way it is in my model. However, the long slope on the locking bar edge in my model makes it easy to lock into place. This model worked for me, so I haven't had a need to modify it further. :-)
Any PETG or ABS (See above)
It's best to print the mounting plate with the ceiling facing side up. That's the way it appears in the rendered image here. This is with the grooves on the sides facing up from the print bed. Printing it this way limits the amount of support material this print uses and leaves relatively smooth grooves for locking the AP in place.
Support material may be required for the slope on the locking bar. I may tweak this locking bar in future versions, based on feedback, but it works for me as-is without support material.