K dog, I've got the one that fits up to 3.5cm and it works fine for boom mount. If you don't have a place to get it from try Adventure cams. The thing I found was I also had to get the "grab bag" of mounts as well, just to get better shots...not required but will give you better options. I don't know about the mast, I haven't got any sails where the mast sticks out the top to mount it to. I've made up a custom mount that clips onto my masts.

www.senaaustralia.com.au/

Overnight delivery and great after sales service, they even replaced some parts for me free of charge.

Cheers...Jase.