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You did a very neat job hand sewing! Looks professional! I have done this but only in a single layer. Yours looks better.
Buoyancy isn't as important as you think. It just needs to be slightly negative. The important thing is center of gravity, which affects angle of attack, which controls rate of descent. In the instructions I make a big deal of altering the center of gravity in the field to suit by sliding the camera back and forth. The pipes stay sealed and full of air. If you want to experiment with buoyancy, you can bring multiple lengths of pipes, or start long and do some cutting in the field. I haven't tried saltwater, but it shouldn't matter too much. If you find the rig floats in salt, you can just let a little water in, or put a dowel down the center pipe, plenty of fixes for that.
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