We are setting up several UAP-AC-HDs for a client in a new fitness center, and needed to hang 2 HDs from the ceiling trusses. So we used the U-Pro-MP https://store.ubnt.com/collections/accessories/products/unifi-professional-mounting-plate mounting plates with some standard electrical hardware and black steel pipe to custom build the mounts -
A standard beam clamp + pipe clamp at the top, and a standard square Utility box at the bottom for the plate to mount to
The mounting plate replaces the plastic ring that comes with the UAP-AC-Pro, HD and SHD, and has a myriad of holes in it to fit a wide variety of US and European electrical boxes. Just bolt it in place to the ceiling truss, feed the cable through the pipe, terminate, plug in the AP and snap it into the mount. Makes for a very simple low cost, robust mount.