Well, so far I've done 5 design revisions, made prototype 1, made more revisions, and I'm currently building prototype 2. As the mast flotation takes shape I uncover problems and get new ideas, but there's still some things I can't know until I test it in the Spring, like what is the minimum amount of flotation necessary vs. how much windage can I live with?
The current design is simple and modifiable: Just a tube of white ripstop nylon that I can stuff various amounts of styrofoam boards into. It will stay attached to the sail because the tube, which folds around the bolt rope and around both sides of the sail and is fastened to itself at the leach, has a hole on the boltrope/fold that captures the top sail slug. The tube can expand and contract to hold more or less layers of styrofoam. Prototype 2 has a max of 6 layers of 0.69" thick boards (4.1") or 19 lbs of flotation. I expect to taper the leach side and end up with 15 lbs. However, there is some possibility that the thickness of the flotation will provide a 'plate affect' reducing the induced drag and increasing the aerodynamic efficiency of the sail area below it, instead of just providing the expected inefficient windage. I'll stick some yarns 'round about it and see...
Below this 'upper' flotation is 2.5 inches of just mainsail followed by an optional flotation to be deployed when I feel like it. The 'just mainsail' is to allow the mainsail to flake properly. The optional flotation works the same as the upper, but it is more aerodynamic in that is is 1 layer thick (0.69") on each side but is about 15" in height giving it about 17.5 lbs bouyancy. The styrofoam should have cuts in it from top to bottom every inch or so to make the board more flexible. That's a cut (3/32 +/- 1/32) every 2 inches on one side then on the other side offset 1 inch and cut every 2" so it bends equally both ways.
I found some even cheaper styrofoam at Home Depot and it's in a much more manageable size: Cellofoam Poly Panel EPS Insulation and a $7.25 package is 6 pcs 3/4" (actually 0.69") x 14.5" x 48" which is 1.7 cu ft! And it's white! (It is not however, flame retardant, which seems strange a building material having less flame retardant in it than humans.) I tested it and it's 60 lbs of flotation per cu ft. I also tested seam glue which holds ripstop moderately well in sheer, and double sided tapes; some are ferocious and some act like 'sticky notes' on ripstop material.