I've used the 2x4 method, two, only attached with a pair of u-bolts through each board. You have to drill, so slightly more work than hose clamps, but I'd have to think more solid.
That was on a old Rav4, where the racks are less than the width of one canoe, and only about two feet apart. We carried a pair of 17' boats. It looked freaking ridiculous and it really didn't work that well in our case, because we were carrying the boats upside-up and filled with gear. Oh, the racks were fine. It was us that was the problem, and a several hours long drive across rutted dirt roads. By all rights, should have been a disaster, but we made it okay.
I would have suggested this earlier, but the legality is questionable, depending on the state and the size of the car. Many states limit how far the bars can overhang.
Free Standing Boat Racks
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