Em
What we did, was to make templates of the carpets we wanted to put into our van and traced them onto the back of some leftover carpet we had, from re-carpeting a room in our house.
Make sure that the carpet is hessian backed and then get the carpet(s) bound by a local flooring retailer (not a major chain).
Ours cost me 50 pence per foot to have bound, and the carpet we used would have been skipped otherwise.
Total cost, for much thicker and warmer carpets than those supplied with the van, and which match the van interior.....