Maybe you can line the U-bolt with a strip of metal to spread the clamping force over a larger surface area? Something in the neighborhood of 16ga, might be able to find something in the plumbing or elec. dept of your hardware store.
Too bad your crashbars don't have a short section of square tube to clamp to.
Got any pictures?
Might also be able to make your own clamp blocks along the lines of handlebar mounts.