If you use styrofoam, like coolers and docks are made of, you'll need to do more sanding than carving, unless you have the wire type electric foam carving tool. The balls in the foam separate out unless you melt them with a wire or by sanding. I tried to carve some with a turkey knif, but it was very tough going, and I didn't want the mess of all the sanding it weas going to take to shape it. You might consider common househoold items and duct tape for making shapes. I find pizza boxes very shapeable for making angular surfaces and corners, etc. Soup strainers come in a few sizes for making rounded shapes, and they're lightweight and cheap to buy. you can leave them inside the tailpiece with little extra weight, if you arent' making a "plug" for a mold. I used an 8 inch one for my tailpiece, to shape the aft section. You could also use thick Paper mache' over a balloon to make a form you could glass over, for a plug, to make a mold. Then you can control the exact size of the sphere. I made a darth vader helmet that way once, using paper mache' and bondo. It would have made a good plug, but I just got tired of making it..back when I was younger. Here's the tailpiece, with the neighbors house asnd the clouds reflected in it. Pizza box, duct tape, soupe strainer...then glassed over, and lots of resin before sanding. A little polyestrer to make it perfect then paint.