One thing I thought would be handy is a gopro mount that plugs into a Planet Bike Super Flash mount since I have one attached to the rear rack on every one of my bikes. Though I'm not sure how secure it would be.
I've been using aluminum for custom mounts (plate, square tube, L shape). My primary reason for doing custom mounts is so I can attach a forward camera to the frame instead of the handbars which adds side to side camera movements.
This one is particularly ugly. It started with two holes spaced for attaching to a bottle cage bosses. Then a hole large enough for a gopro tripod mount attached to a 1/4 - 20 set screw, and 3 holes for a k-edge flat mount to allow mounting in 2 directions. This week I used the 1/4" hole to attach it to a Minoura large camera mount with a wingnut. The Minoura clamp is very secure, just don't forget the protective plastic strip. Tonight I made a longer but more compact version using L shaped aluminum to get the camera out past the shifter cables and eliminate the need for extra plastic mount pieces. No room for the wingnut so I have to go get a 1/4 bolt tomorrow.