After some discussion with Aerotronics regarding interference, both physical and RF, I have decided to put the VP-200 boxes just aft of the firewall between the deck ribs. I started by making the basic mount out of alclad sheet with a Z-bend at either end which results in a standoff of about 3/4" both to keep the boxes cool and allow room for nutplates. I then match-drilled the template box Aerotronics sent me to the plates and discovered that I wouldn't need cross-stiffeners as the box does this quite well. I installed the nutplates for the box and then screwed the template to the plate and match-drilled the plate to the firewall (orientation of box has the stud for power to the left, so the logo is upside down). Then I installed more nutplates and repeated with the other mount. In the last picture you can see the right box in place on the firewall. Using the firewall stiffener below the boxes as a measuring point, the box is just above the stiffener, 11" from the side to the outboard edge of the box and the box face (with the logo) is 17" forward of the panel. The other side is the same and there is about 5" between the two boxes. In practice, I'd never be able to reach under the panel to get at the screws for the boxes, so the procedure would be to unscrew the mounts from the front of the firewall to drop the boxes and allow removal of the wiring. Installation would be the reverse and require someone inside to position the box/mount combos while another person inserts fasteners from the firewall front side.
Rivet count: 48 Total: 15419 - Plus 549 done at Van's