Pictures of both helmets? your 661 and D2 udner visor? im curious.I've mounted my GoPro Hero to the 661 Evo helmet using curved base. I also used layer or two of thick aluminum sticking tape and rubber thingy to reduce rattling to zero
I made my own chest mount using piece of GoPro's box and pieces of Headlamp-Style Head Strap and piece of dense foam. I don't have buckles on it, so wearing it is a bit tricky I use Vertical Surface J-Hook Buckle to mount camera upside down and flip the image in VLC for watching.
I've bought HR (German company) suction cup mount for GPS and combined it with GoPro mounts. Used it in the car on on motorcycle, so far with success Cost about 10$
I plan to make pole mounting device. I think it will need some counter-weight.
PS1. there is one problem I have to solve. GoPro's tightening bolt hits leatt brace, constantly. It's pretty annoying. I have to replace it with bear bolt, I guess.
PS2. My buddy mounts my GoPro on his D2 helmet just under the visor. It give nice, centered perspective, w/o any part of the helmet in the camera's view.