Looking for ideas. Portable, inflatable camera float

I like all the ideas, but the polystyrene the best, simply because I could probably build something with excellent floating characteristics that also stayed out of the way.

Managing it on dives is the issue, but if I limited myself to over-unders at the beginning of a dive, I could simply toss the float back on the boat before resuming the dive.

One of the issues I see with floats is that most would be unstable, though the enterprise tends to be unstable.

This was the first over-under I ever tried with this rig, in 2016.   I was at the end of my last dive of the trip, and when I came up, I saw a waterspout.   Finning furiously, I managed to heave the camera up out of the water in the right direction, pressing the AF-ON button and hoping for the best.   Didn't get much 'under', but I was actually trying to get the 'over' here.   The effort showed me how hard this would be.

I've been diving in the same place each September for the past 5 years (Reef House Resort in Roatan), and this year I decided to try something new.   For my check-out dive, where I do a shore dive the afternoon I arrive, I thought I would simply stand in a shallow area, and get a shot of the Reef House over, and their 'pool' area under.  I would dangle both strobes downward and light the under area.

On a calm day, this would have worked OK, but there was so much wind and swells nobody could go in off the end of the pier as usual (and then come back in to the shallower area).   I went in the shallow area, and could not get out to the end of the pier due to the waves, so I tried the over/under.   Too much wave action to get the shot as I was barely able to stand more than a second.    Here is all I got:

Just viewing the LCD can be a problem when half of it is underwater, and bright sun is out.   The camera really needs to be in a neutral weight configuration for this.

Hmm.  Maybe a bag of styrofoam beads...