I was just thinking of this and came up with a great idea (if I do say so myself). Just make up the tray (http://www.equipt1.com/h/components.../product/5bd3d59f179c10e63f39e84aa5fbd06e.jpg) and use a regular strap to hold them down. That tray shouldn't cost a whole lot to make (could probably just use scrap?). If you have a welder weld the 4 corners, if not, rivet them. I have a Front Runner rack (cross members run side to side, not front to back) so all I'm going to do is take a piece of flat AL, bend up one end and bolt that to the rack, wedge the cans between the lip on the rack and the bent side of the plate, then ratchet them down with the D-rings tight against the side of the cans (http://www.equipt1.com/h/component/...egory_id,159/option,com_virtuemart/Itemid,31/). Will be plenty strong, and won't cost me hardly anything. You would need the full tray since you wouldn't have side support and the cans could just slide sideways.
Clark
I was thinking about doing the same thing. I was just going to mount mine straight to the deck of my roof rack, and run straps over the cans.