Back in the 80's I glued a dummy canvas roof to O gauge wagons. I still do it where appropriate today. And the PECO van is no exception. Simple enough. Take a single square of bog-paper and de-laminate the 2 plies so you have a single piece. Plonk over your roof and with your solvent of choice, wash liberally along the ridge. Then making sure it stays un-ruffled, work down to the eaves. Trim with a pair of nail scissors leaving a couple of mil to wrap round the edges. Keep trimming and solventing until complete. Paint light brown as a base coat and then your chosen top coat.
I tend to do the sticking in stages 1. to let it not get too soggy and 2. to stop to much fume build-up as you have to slosh quite a bit of solvent about.
Andrex is best avoided unless you want little puppy holograms on your roof.
If you're clever you could pre-scribe planking onto sections of the plastic and create a tear/flap on the canvas to show damage.