For the camera mounting device I use a mini-tripod with flexible legs, three pieces of bicycle tube and cable strips. The pieces of bicycle tube are cut to fit the length of the tripod legs. These serve to avoid the tripod legs from moving. I have an Omer Cayman speargun and the legs of the tripod fit around the handle of the speargun. The feet of the tripod legs are then held together with strips.
I am quite fond of this arrangement, because the flexible tripod legs make it easy to move the camera to fit the angle from which to shot the videos. However, when a shot is released the camera may move.