I'm thinking about ideas on how to ensure that the cameras will always be in the same position relative to each other. That is, the cameras can be attached to the payload one day and attached on a different day and they won't have moved.
I'm currently thinking about a custom slab (a piece of lexan, maybe) with alignment pins that the camera mounts to. The pins slide into hardened holes in the payload container.
The payload container can still be a styrofoam cooler with this method. There are chemical that will cause reactions with styrofoam that will cause the pellets to shrink and harden.