Settled on a location for the ELT and corresponding antenna. Prior to routing the control cables, I foolishly tried to mount my ELT dead center down the rear of the fuselage. In the 1st photo you can see remnants of the platform I had built. That location is about as far back as I can reach through the "hell hole". There will be plenty of clearance for an antenna connection though, so I fabricated a reinforcement plate and mounted the antenna there instead. I was even able to reuse some of the holes I had already drilled in the skin.
For the ELT itself, I chose the area behind the flaps servo. That area of the bottom is pretty stiff and I doubt much more integrity would be obtained by attempting to fabricate any other type of mounting. There is just not that much structure to work with in the tail section. Nevertheless, I added a doubler plate for even more rigidity to the bottom and attached the ELT caddy. The tape in the photo is to cover the physical switch and cable connector until I am ready to attach the cable to the dash control and buzzer and the antenna.
Later in the evening I went to the NOAA website https://beaconregistration.noaa.gov and registered the ELT as required.