Personally I would mount the battery under your rig and add some buoyancy to the top.
Here are some pictures of a system I've developed which I hope are self-explanatory. It involves attaching a small buoyancy chamber (ie bucket) to the top of the rig in such a way that it can be moved forward, sideways, or to the rear to provide buoyancy exactly where required. Adding/releasing air from the bucket and adjusting its position allows you to have your unit neutrally buoyant and sitting straight and level, or alternatively at any angle you want, at any depth. Releasing all the air allows you to place your unit on the bottom, nice and steady for those closeup nudi shots.
It looks ugly, but it's cheap and it works!
IMG_1706a.jpg 66.48KB 10 downloads
IMG_1708a.jpg 50.08KB 10 downloads
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