I have made bombs for several different scale models. I have been making molds and then lay up two or three layers of 2 ounce cloth. After curing I trim the fiber glass shells even with the surface of the mold and then fill the shells with two part polyurethane foam. Then I trim the foam to the mold surface and join the two halves, still in the molds, with Gorilla Glue. This gives a perfect fit to the two halves. Then I add fins made of polycarbonate plastic. Photo of 1/10th scale WWI British 1650# bomb. The fins get broken over time, but I have never broken a body.