Polyisocyanurate vs syntactic foam for flotation



Hi all,
this is a bit of topic, but I am putting together an animal sensor tag that collects movement data on large fish in the ocean, detaches automatically (through a galvanic release) and floats to the surface for subsequent retrieval with the help of a VHF transmitter.

For that purpose I am designing a flotation device to overcome the ~100 g of tag weight. I am considering to get some syntactic foam but I am worried about the difficulty of machining it into the specific shape I am thinking (likely something torpedo shaped with an opening of the sensor unit and the antenna of the VHF transmitter). I have made up my own syntactic foam using epoxy and glass microspheres before, but given that we are only working at shallow depths (< 200m) I was thinking I may get away with polyisocyanurate foam for flotation.

Does anyone here have experience on how polyisocyanurate foam behaves at shallow depths up to 200m? I appreciate any guidance!