Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
Ability to post and comment on topics and discussions.
A Free photo gallery to share your dive photos with the world.
I wrapped mine in some old 7mm neoprene wetsuit fabric to get some buoyancy out of it. I am also going to try putting a tub inside the arms and filling with some expandable foam as well. You could also use some STIX floats on them as well.
If your main goal is better support for heavier weights, then you should think about ball arm systems.
It seems that most video people start with flex arms and most photos use ball arms. I have used both (for both) and feel ball arms are better. They provide better stability and can provide better coverage - if you have enough joints.
Flex arm systems have a minimum bend radius. A dual joint ball arm system can easily beat a flex arm.
The lightest (buoyancy) arm system I am aware of are the resin based Sea & Sea arms. But they use a larger than standard ball, 27mm instead of the most common 1 inch ball (approx 25.4mm).
I've used flex arms for 15 years - they are much quicker to reposition and I have never felt any limitation to lighting angle using them. I also have ball arms and while I don't have nearly as much experience with them, I prefer the flex arms.
Maybe try polyethylene foam tubing, the kind they sell at hardware stores to insulate pipes. It usually has adhesive on it and comes in a few different t sizes. If memory serves me well, it's also cheap.