Most gutter-mount rack systems are pretty expensive -- often a couple hundred bucks just for the mounts, and the vans require extra-long crossbars. The cheapest DIY option I found is this:
$60 for a set of four. They're designed for a Jeep XJ but I suspect the rain gutter profile is not that different.
After some quick measurements I think the HF #66983 "500 lb. Steel Cargo Carrier" is going to be a better fit than the ATV carrier. The van roof is wider than I'd have thought. I'm going to hold off on ordering any kind of basket until I've mocked up the gutter mounts, though. The cargo carrier is massive overkill strength-wise but I haven't found a cheaper cargo basket yet. HF does sell a 150 pound capacity rooftop basket type carrier, but it's very narrow (36 inches) and I'd prefer to get closer to the full width of the roof.