zipnz1,
I have been making bombs by the same molding technique you have posted. I have had a lot of success joining the halves with the following method: I use "Mountains in Minutes", a two part polyurethane foam that is sold to the model railroad guys. After molding the two half shells in fiberglass, I trim them flush with the mold then pop them loose. The shells are then returned to the mold. I mix up some polyurethane and pour some into each half of the mold then clamp the mold halves together.(You need dowel pins to align the molds). I leave a vent hole at the trailing edge of the bomb. The polyurethane will expand and glue the two halves together in perfect alignment. When the cure is finished(just a few minutes) open the mold and remove the assembled bomb. Then I drill a couple 1/4" holes and epoxy in short lengths of wooden dowel as screw anchors for the attaching bracket. I have been making the shells from two or three layers of 1 1/2 ounce cloth. the bombs are light enough that I have not broken one yet. I make the fins out of glass too. I laminate a couple layers of glass cloth and resin between two peices of plate glass then cut fins from the resulting homemade "G-10." The circular fins on British bombs can be made with fibreglass and resin over a mandrel (Broomstick, pipe, bottle, etc)
Chuck