I would take a single piece of ply wood that runs the length of the trunk. There is enough room to screw down the sides by the wheel well, and then mount the tubes to the plywood. You can sort of see what I'm talking about in my setup.
http://photos.yahoo.com/bc/svt20002001?d&.flabel=fld1&.intl=us&.src=ph&start=1
