Materials for lightweight box on a bicycle trailer

Hi, I've been considering various materials for the construction of a lightweight lidded box to fix onto my bicycle trailer. I started with the idea of using 3mm plywood over a timber frame, which is still a possibility. With plywood as the benchmark for durability and lightness I've also been looking at "Foam-X" "...a featherlight material sheet with very good flatness. The sheets consist of an elastic, closed-cell structure core of expanded polystyrene and strong, coated liners." Which I found an example of here:

The strength and durability of a structure made from this board may be increased by first making the box then covering the whole surface area with substantial gauge of aliuminium (uk) foil.

Please can anyone tell me if there is a more appropriate type of sandwiched foam board for this purpose and whether the aluminium foil covering will be the best material to cover it with, considering the pupose of the box?

TIA Mike