I am wanting to build a box on my trailer 7x4.
Been looking at using upvc sheet. Have been quoted £36 for aa 8x4 sheet 2.5mm thick.
Is there any cheaper options that are not wood.
Matty

I used 2mm ali. Still in good shape 26 years later.

Check out your local AALCO depot - they will stock GRP sandwich panels as used in dry freight commercial vehicles (probably a bit much for what you want), or my recommendation that would be one of their aluminium composite panels (thin, lightweight, but very durable). Won't be as cheap as your PVC sheet idea, but will look and last infinitely longer.

Aluminium seems a good bet - easy enough to cut and fix and durable too.

plastic covered steel as used on buildings lodes of colours too