well.. you guys have seen my crap..since you have the workshop, i'd recommend building your own.
anything you buy will:
1) not be as space efficient as a simple rectangular box
2) over designed for aerodynamic performance
3) made of less durable materials
a simple box with a simple fairing will not only be plenty sufficient, but far easier to get in & out of, & far more customizable..
most rooftop cargo boxes have only a hinged lid, so getting shit out is a bitch.
if you have time & resources make your own, side it up with HDPE or vinyl/aluminium siding, and customize your access & storage.
put all the shit you use least up top.. jumper cables, spare battery (perhaps), clothes for other climates (no point having inulated boots downside in the desert, or shorts in the high mountains).
mementos & whatnots can go up there, too, as well as spare water, your air compressor, full journals, unused art supplies, a bit of emergency food, spare gas, odd tools & odd parts.
generaly speaking, when you van/vehicle dwell, you don't carry much that isn't essential or of immediate use at any given moment, so stowage is critical to avoid that crawling up on top of your rig moree than needed.
in any case, you WILL be, so
a ladder directly accessable to a side hatch, or a hatch directly reachable from your existing ladder is critical.
your rig is big enough that you probably don't need to worry about being too top-heavy, but it's a consideration that has to be in you mind.. also, the extra side surface area in wind will have some effect on handling.
you can get a lot of room up there..
it may be quite possible, if you find one or two of them cheap/free to utilize a truck topper or two.
i know from experience that putting one of those fuckers on top of a van is def a two person job.
if you're creative, i think you could make them look more in place than just held on with ratcheting tiedowns : )
BTW.. make sure you have a 6 pack of those fuckers to at least start with..
the remainders go up top, too..
on your rig oppers will need a bottom put in place, since you can't seal them directly to the roof..
anyways.. at the MacDonald Pass vista in Helena NF, MO..
snow & mist & sunlight
& thinking of you guys..
c