I would think you could use a yakima Q tower legs, and modify a set of Q-clips to clamp on the outside edges. If you don't know what a q-clip is, they are car specific attachement points for the yakima q-tower system, but you'd have to modify a set to hug the side rails of the tub. The Q clip for a Jeep cherokee would be a good one to start with, or you could just go to a store that carries them and go through the boxes until you find one that looks like a good starting place.
It would be removeable and shouldn't damage the paint if you pad or use some type of cushion tape on the Q-clip.
P.S. they sell for pennies on ebay.
It would be removeable and shouldn't damage the paint if you pad or use some type of cushion tape on the Q-clip.
P.S. they sell for pennies on ebay.