Go for it! with the added cabin cruiser type structure; 3mm plywood is easily formed and bent to the required shape.
Around here in Norwich and the Norfolk broads area I have recently seen quiet a few Volkswagen T2, T3 and T4 vans that have been converted using the top cabin structures from broads cruisers, and I think they look great.
You could even add a fold down canopy (like the ones on most broads cruisers) to the rear of box to allow Mickle junior to get in and out and to provide a some weather proofing.