Another option that could work for a lot of folks:
FourTreks
I talked to the owner on the phone this week, really friendly guy, and he says he has a specific mounting option for MaxTrax. He doesn't actually have them on the website but you can probably get an idea of how they work from the product in that link. Basically, he has multiple sizes of tube clamps with a bracket that attaches which has holes spaced at the proper distance for MaxTrax pins or bolts/wing nuts. He doesn't supply pins/bolts. I believe he said $120~ for a pair.
Other products he has for sale could be really useful for a lot of us w/tubular roof racks/baskets.