You can make floats using divinycell as suggested however as the material gets brittle you will need to finish it with epoxy glue all over to seal it. One of the other challenges is to make a shape that goes around the arms. Unless you have good cutting tools and a lathe it won't be possible so you will need cable tyre to fix it. It works fine until 40 meters with minimal buoyancy loss. I wrote a comprehensive comparison of commercially available float systems here
Divinycell (high density PVC foam) is easy to shape with basic tools - no need for a lathe. You can also glue pieces together, so there is no need for cable ties. I agree that it has a soft surface that needs to be protected. Rather than using epoxy, I sprayed my floats with a heavy coating of "Rustoleum Leak Seal" which gave a tough flexible finish. After 60 dives, often below 40 meters, the surface looks the same. With floats that were formed by glueing together pieces of foam, I never noticed a loss of buoyancy at depth. I hope my floats on a string will work as well.
I would like to encourage DIY types to forge ahead.