Another couple of options (assuming you are not going to get it wet).

1, Use circular tube Aircon ducting (can be bought in a number of sizes up to about 2 foot Dia I think) - I'm planning to investigate this for my next "box" as it is light and fairly strong (being round) and you can buy standard end caps.

2, Use large scale Aircon ducting (rectangle) - hiever than the circular, but will fit the trailer better.