If you build some matching mold boxes, with 1/4" plywood bottoms and 3/4" x 3/4" borders, spray the box insides with some kind of a clear acrylic sealer, and mix some yellow carpenter's glue into you POP just before you pour it into the mold box, the molds will be strong enough to clamp, and will last for years. I have some that are more than 10 years old and they still injection pour just fine. I line the bottoms of the boxes with fiberglass drywall tape, for additional reinforcing, and put some 1" finish nails around the inside of the borders to help anchor the POP, but I've found that, on the two occasions I had to remove the POP to remake a mold half, the carpenter's glue sticks to the mold boxes very well by itself. Be sure to mark the cavity locations before you install the 1" nails so you don't drill into them when you drill your 5/8" sprue holes. I use cut off 8D nails to align the two box halves, and clamp the halved together after they're poured and cured to drill the 5/8" holes with my drill press. Try and keep the baits 1/2" away from the borders, so you can carve tapered holes from the 5/8" sprues to the cavities.