I tend to think that walking across town with a bass, putting it in a cargo box or just putting it into a hot/cold car are all going to pose the same unavoidable temp/humidity shift, which in a decent bag shouldn't be a problem. I mean, if just transporting an instrument to a gig is going to cause damage, that's not a very serviceable instrument. It's not like were talking about how to transport a bass coast-to-coast or from BC to Mexico after all - rather, just how to get around town with a bass or two and not have to drive an SUV.
Anyway, I think I'll stop by the "rack shop" with a bass and see about that Yakima box, unless somebody's tried it already... hopefully whatever bass I take won't explode on the way over there.