I'll try to explain my solution, but don't have a picture handy. If you're interested, let me know and I can probably get one this weekend.
RV6, tipup...
I took the tray and mounted the front, open end of it to the flange on the bottom of the subpanel, co-pilot side. I built a simple triangular brace that I riveted to the stringer that goes from the subpanel to the firewall. The brace extends below the stringer, aligned with the back of the tray, which it attaches to. The transponder is out of sight and far enough forward so that it doesn't get in the way, but is just low enough (and slightly angled) so that it will clear the bottom of my panel to slide it out of the tray if required. It is also forward CG in that position, and the wire runs are easy. Behind the baggage bulkhead is well aft CG (good or bad, depending what you need) and the wire runs get harder.