Here is my mount
Following is my construction sequence.
I used 2 pieces of 1/2 X 1/2 angle cut to match the length of the rear cargo area upper shelf. I match drilled three 8-32 screw holes in the angles using the holes in the edge of the cargo deck as a drill guide. I then cut some 1/8" by 2" wide stock to span between the two angle pieces when placed on the upper and lower longeron. The next step was to drill through the lower angle and attach the two pieces of 2" wide aluminum at proper spacing to attach the ELT mounting plate. I installed K1000-8 nutplates on the angle to allow me to install the 8-32 X 1/2 inch cap screws that I used to fasten the angles and 2" stock together. I then fastened the lower mount angle and the two pieces of 2" stock to the lower longeron using the same 8-32 screws used to install the cargo floor. I then laid the upper angle on the top longeron and clamped it to the 2 inch wide stock. With the upper angle clamped to the stock I removed the entire assembly from the plane. On the bench I then drilled the 2 inch stock and the upper angle, installed nut plates on the upper angle and re-assembled the mount completely. Next step is to re-install the assembly in the plane again using the 8-32 screws to hold the lower angle in place on the longeron and cargo floor. With this in place you can use the upper angle which is fairly rigid as a drill guide for drilling through the upper longeron with a 90 degree angle drill. Fasten the top angle in place with 8-32 hardware.
The only real down side I see to this is that in order to access the rear mounted battery it will also be necessary to remove the ELT and mount. Other than that it's a pretty clean installation.
You must be careful in drilling the upper Longeron the keep the holes centered in order not to compromise the Longeron.