Are these boxes some kind of universal fit with roof racks? I have a BMW 335 with a "factory" roof rack (been a long time since I bought it but I seem to recall it's just a rebadged Thule or Yakima, et al) -- BMW has a pretty nifty ski rack that I've been using but I do like the idea of keeping the gear in there for the season instead of the re-load process, not to mention the salt and the grime. But no idea what boxes would work with my roof rack, and which wouldn't, but maybe it doesn't matter -- they'll all work?
My Thule has sliding clamps, four of them, that adjust fore and aft. They have good range.