Just stumbled on this... never heard of rockport van boxes (in comparison to supreme and other well known manufacturers), but it would be a pretty interesting starting base... Not to mention the box already has quite a few interesting features... nice rear door, multiple windows, stainless steel sink & grey water tank, 80" interior standing height.
In my brief research I also discovered that Rockport makes 10ft, single rear wheel boxes for the transits, something I've been looking for, but yet to find. Would be super interesting to see one of those... I'd love the potential better gas mileage of the Transit option engines vs. econoline engines. Biggest downside would be lake of 4x4 conversion options, of course.