Tested another version of the swivel mount during the first day of the year, this time using a wheel-bearing assembly (razor scooter), instead of a wheel caster. I thought it would get rid of a lot of "shake" I was getting from engine vibration, since there is a lot of play in the caster wheel. Looks like it didn't really make a difference, and most of the excess vibration from the too-stiff motor mounts was getting absorbed by the new Hero 4's super wide angle anyway. Next project is to replace the motor mounts back to rubber from poly, to smoothen out the bone-jarring idle we started getting after doing the 1.6L 16v motor swap. Or, what about getting a gimbal? $!