Greenie,
ping me a pm and I'll send you some photos of the system we used a while back with the multiple units we were using for work... and the occasional windsurf expedition

We found the best system was from a bike lock for a push bike.
They had really strong mount points and the shape suited the cameras (bullet) style like a glove based on the standard shape/diameter of a push bike frame.
Once they click together they will not come apart and there is an inbuilt safety/fail safe... which means you wont be losing your new camera !
We modified the gath by drilling out the ai'r holes' in the tops and sides.
You can then pull the foam out of the inside of the helmet and push the bolts from the inside out.
Get some good quality stainless bolts with the flattest head possible.
It's a bit fiddly but the end result is super secure AND there are no sharp/metal bits inside the helmet.
Once you push the foam inserts back in there is plenty of protection for your scone but we thought flat head screws were still a worthwhile investment.
Kev