I did the exercise ball enclosure thing with fiberglass and resin. You must wrap the ball in plastic shrink wrap before hand to keep the fiberglass resin from sticking and attacking the ball. A gel coat will help alot here and give you a nice clean finish inside the sphere. Also make sure the ball's air valve is accessible via a small 4-5 " opening left uncovered. You could of course also use epoxy resin instead which is safe for the ball and also has less odor, but you'll need some form of mold release ie. cured PVA or white wood glue. Once you have a few layers thickness of fiberglass, deflate and remove the ball from inside through the small opening, then enlarge the hole to accept a mounting flange for your driver. Finally add a few more fiberglass layers until you end up with about 1/4 to 3/8 inch thickness depending on overall diameter of sphere.
A spherical enclosure requires special dampening strategies to avoid strong resonant standing wave modes from occurring, as you will encounter completely identical wall to wall distances inside being its sphere shaped. A larger balled up variably density bunch of dampening materials is best for this suspended in the center of the sphere a few inches away from the driver rear surface, as well as some heavier density dampening lining the inner walls.
