wetsuits, while having some floatation affect are not designed to float, they are designed to trap some water and as the body warms that water has an insulating affect on the body.
My Tiger Shark mast (twenty years old at least had the tip sealed with foam. Still in good order and mast well sealed.
oam in mast will not help your problem, you want more floatation not different floatation.
I know he idea of changing boats at this time might seem tired but a lighter platform might negate the need for a trainer bubble at all.