Sounds like it will work. I just got off a San Juan trip and didn't rig as though it were the Colorado, but even still, if it fits, it fits and it's lashed down. I don't think the Scepters could produce enough friction or hard angles to worry about your raft. FWIW, I use two 10 L Sceptre and one 20 L Sceptre. The small ones go on top of the tube flat next to the cooler I use as a seat (on either side). There's a slot on my cooler on each side, with a 3' strap and one wrap around the frame and a wrap around the jug handle --- it's on its side, but not going anywhere. I try to keep the weight in the middle of the boat, so you're on the right track.