I-Divesite Ball Mount for GoPro

This will turn your GoPro into a ball based socket. That way you can mount it on all armball based systems! Really good quality, with O-ring - to keep stability on the armball, so your GoPro does not get out of sync.

If you want to mount light, as well as the GoPro on your housing, then you might want to consider, to use a Tri-Clamp - or the MP-clamp system.