In planes with lots of internal room, I build a lopsided "A" shaped mount from lite ply and Hysol that to the floor or sides, then strap my UAT to that. Here's a pic from my Boomerang XL that use this type mount.

I have also velcroed them to the side wall, a la Bandit ARF, or installed them vertically into precut holes in the equipment trays (a la KingCat).

Barry