Some take a plastic container like Tupperware and enclose it and then silicone all the entry points, but again I would still be careful, but at least if it got splashed it should be ok...
Your safest move is to get a detector that is made to hunt in the water. You hunt the ocean long enough and no cover will save you when that rouge wave knocks you on your A$$ Home made cover..inexpensive new detector after being dunked $$$$