Update 20180614: v4 is here! https://www.youmagine.com/designs/action-camera-diver-s-handle-4-aussieland-edition

That's the 3rd generation underwater camera handle I made (1st gen is here: https://www.youmagine.com/designs/gopro-diver-s-handle , 2nd gen. is here: https://www.youmagine.com/designs/gopro-diver-s-handle-v2).
People loved the previous ones too (almost 4k downloads so far) but first of all they had GoPro only mounts and they required some extra assembly time. This new one is a simplified version and dedicated to my upcoming new aussie diver friends :)

Print 2 pcs from the rod, 2 pcs from the spikes and a sole. The sole is a bit big but I could print it by turning the object 45 degrees around Z axis on my Prusa i3 MK2.
The last rendered image explains how to assemble the handle. The 1/4"-20 UNC bolt is fixed to the bottom of the rods by an 1/4"-20 UNC nut pair from underneath. You have to heat up all the other nuts by a blowtorch to fit them to the sole and to the spikes.

At the end you will have 4 open 1/4"-20 UNC threads what you can use as mount point for misc. photo gear, e.g. dive lamps.

If you want to leave the camera in the bottom sand for a while (e.g. because you will need to make a video from the very shy sea eels) you can reverse the spikes and screw the rods to the bottom of the sole.

The thread in the rod is 7 mm long M18x2.5

Update #20180320: I have uploaded three more "spikes": two with holes to 1/4"-20 UNC threaded inserts and another one to fit a light using the amazing Dinkum FlexiMount joints: https://www.youmagine.com/designs/dinkum-fleximount-1-4-1-5-unc-joint