Think you know FG?

k.. this is a long shot, but it reminds me of like 1st grade school.

get a balloon, blow it up to your desired size, then paper mache it, and wrap it in a layer of tin foil, then do your fiberglass work.

you can make it any size you want while keeping a realtively good shape (the paper mache also allows you to mold it a bit)

my really f*cked up 2 cents :)

FYI: the walmart ball mentioned earlier is not a regular beach ball.  They are extra heavy so that children can jump on them and stuff.  The plastic is about 3/16"-1/4" thick, and they will work perfect for what ever it is you're trying to achieve, provided you can match the correct size. 

PS. don't ask how I know that lol.

you wanna know how to do it? I'll tell you how I do it. get your self a big bouncie ball at walmart. get some two part expanding foam. Let all the air out of the ball cut a hole pure the mix in it. LOTS of mix. work the ball out as it expands press it down so the foam mix fills every curve. fill it to the top let it dry wrap it with tape and glass it. the foam keeps the rubber ball from deforming.good luck,have fun and don't get to excited over your big ball. oh yeah if you want the foam out cut it in half and patch it back together when your done.

Actually, the ball idea is a great idea, get yourself 2 balls one small one thats cheap and expendable the other one the correct size, test your resin on the small ball to see what happens, use small amounts, mix it cold, mix it hot and mix it normal to see which works better, the thing you really are wanting to avoid is well 2 things, 1 a chemical reaction with the rubber, and 2 a reaction from the heat of the resin setting, both of these will be very bad and cause horrid end results, mixing your resin so it sets faster will produce more heat which may eat thru the ball that much faster, another thing is that fiberglass most likely wont stick to the ball anyway so the actual chemical reaction with the rubber probably wont happen and the ball will come out rather easy as i've seen resin stick to plastic like crazy but never stick to a trailer tire or anything of that nature if your really smart you'll make a 2 peice mold from this pull as the inside will be just about perfect, make a few little repairs a new coat of resin on the inside to seal the mold and it should be great, then you can make as many of these as you want ;).

J~