Is a foamie cargo van topper a bad idea?

I'm looking at making a bid on a cargo van delivery contract that my van just falls short on the storage space requirements. The hit on the initial cost and ongoing fuel economy to upgrade to a full size van would be pretty painful. Am I crazy to want to try this? I've had a look up close at a GMC safari someone built a plywood camper topper for and there didn't seem to be to much too it. With fiberglass cargo toppers costing nearly $3000 and my van (2009 pontiac montana SV6) being a shorter size than most toppers I could hope to find at a scrap yard. I'd be using it exclusively for the added cargo/standing room so I would probably cut the van roof a foot or so back from the drivers seat and use that portion of the roof as a storage cubby and have the added structural strength in case of a roll over.
Any thoughts or advice? Stick with a commercial solution, go for something heavier or it will work just fine?

