I have black self-adhesive carpet tiles from Poundland! £1 (funnily enough) for a pack of three. A tenner was enough to do the back of the van. Just put it straight on top of the previous carpet. Does the job nicely.
I'm still undecided if I should put the original interior back in or custom build something, I want as less wood as possible to save on weight. Might just put a decent custom double bed in the back and 2 seats ???