abonnin, haha, but the first part of the saying is rode hard, the fun part. Kind of speaks to my take on the boat, use it. Also, my boat was a Rep's demo so it's not exactly pristine looking.
My thought on stopping the slipping was to try to avoid drilling the crossbars. About the only thing I had come up with was a sealant I use called Through the roof, very strong stuff. I would have put some under each clamp before tightening and then a bead on each side of the clamps. I think this would do it.
My other concern with this would be that all of those forces, which are now being dissipated in several areas, will now be concentrated on the hull at the location of the clamps. Not sure if this would be an issue but I'd rather have some slop in the crossbar than a cracked hull at the clamps.