I have used foam core sheets to construct high altitude balloon payload boxes for many years. They are inexpensive, lightweight, and very durable. It is easy to cut ports and holes for cameras, instruments, antennas, and wires. And it is no great loss if they are damaged during recovery.
An excellent description of foamcore construction techniques by Mike Manes (W5VSI) is available from EOSS. http://www.eoss.org/onlinepubs/construction/foamcore/foamcore.htm This is where I learned from.
Here, I would like to describe some ‘features’ included in our/my construction. Recently, I’ve begun including a back door to aid in manipulating and connecting wires and hardware in the box..