I made a cannon for Halloween last year and needed some cannonballs to complete my pirate cemetary. I was going to buy styrofoam balls but realized I might be in a high wind location. I figured something would come to me and several days later I found giant Christmas ornaments at the Dollar Store! Thankfully they had already put out Christmas stuff a month before Halloween. I bought 4 of them (styrofoam balls are never that cheap), sanded the mold separation lines down and painted them flat black. I turned the little ornament hanger "nipples" inward and hot glued them together in a pyramid. Done!