In my shoppings for used vans, I have seen several with contractor-style roof racks. They clamp onto the gutter with clamps that def do not look custom made for the model of van. They don't fit that well. This makes me think they might be available OTC in the states, at home depot or some such.
I have no idea whether this is helpful. They do look sturdy, though, and could likely be modified.
IIRC Mex is coming to the states soon for more maintenance work / upgrades.
3/4 timing in a DIY van conversion. Backroads, mountains, boondocking, sometimes big cities for a change of pace.