JR,
My experience with pop is that it doesn't like to be dropped, no matter how thick.
I actually line my plywood boxes with fiberglass drywall mesh tape, to give it some reinforcement. That way, I can get away with thinner molds with them being too fragile, but my boxes are the major reinforcement.
I always coat the box with some sort of spray-on clear, so the water from the pop doesn't reach the box before the pop dries.
I still haven't mastered laying my soft plastic masters onto the open face of the pop mold without having an irregular face when I'm done.
I know the smart way is to glue the masters to a flat surface, glue my 1X frame around them with hot glue, and then fill the frame up with pop. Once it's full it's a matter of laying in the mesh, and then pushing the plywood back down onto the wet pop, forcing the excess out of the sides, and then attaching the plywood. But that is a method I haven't tried, yet.