I sewed mine (the pattern is on the Hill Longbowmen web site somewhere), but the Great Stuff should work well and be less work... for those that aren't antique sewing machine addicted. I'd say pick a light weight kid's play ball rather than any kind of sports ball, though. Go for a thin wall... and get a couple. It might take a try or two before you get a feel for the right amount of foam to squirt in. That stuff expands a lot! Another, even simpler idea would be to just get a cube shaped cardboard box and fill it about 2/3 full of foam, leaving the top open. It'll expand up over the top. When it's dry, saw off the excess expansion to get your cube back, then saw off the corners to get an octa-whatever. Put a loop of rope in the box when you squirt the foam in and you'll have a rope to throw it by. Heck, might go out and do that myself later!