I continued with the rheostat module mounting brackets again tonight. I riveted the front bracket to the airframe which took some setup and contorting to get the bucking bar in and drive the rivets.
With that done I mounted the TCW bracket back in the airplane. I then checked the fit and clearance for everything by installing the rheostat module and then putting the TCW backup battery in its mounting place.
Everything seems to fit well and no clearance conflicts. I made sure it would be in a place where none of the avionics or trays would interfere or cramp each others space.