Is a doubler necessary or optional for the transponder location under right baggage compartment? Plans included one for com 1 and 2. Inclined to not worry about it.
I also built and installed a doubler for the GPS antenna on top. I installed it with the small head rivets with a slight countersink in the skin to avoid having to dimple. Usually I use those NAS rivets for nutplates. Worked out great.