I just slide the crossbars together so the sidewall sits on 'em then tie it down with a heavy-duty ratchet strap (motorcycle tie-down). I've had some problem with it shifting on the trail and slipping off one of the rails and down onto the roof, but someone recently suggested the rubber waffle-style toolbox liner you buy in a roll at Home Depot in the Tool Corral to wrap around the crossbar to provide anti-slip. Sounds like my solution.
Brent