giant ice box /cooler -suitable fastening materials?

re:   Giant cooler Project

I propose (still a proposal!) to use 8mm PVC Insulation panel(plastic) to which the  polystyrene will be bonded. In view of the ‘huge’ size of the boxes – 85cm high by 85cm wide by 130cm long, they will be assembled /disassembled at the location of use for ease of transportation and storage. The challenge   now is the best method of fastening the 6(six) members i.e. 2sides, 2 ends, plus top and bottom panels. I’m worried bcos the fastening must be such that leakages must be totally eliminated else insulation stands compromised.

POSERS

1.       For the 4 standing members i.e. 2 sides and 2 ends, will a ratchet strap (or 2) do the job? Any better ideas especially as I propose to lock the fastener?

   ,   2.    How can I firmly secure the top and bottom panels to the standing panels? The top panel     will of course have a door about 1/3 the size of the top panel with a key