1. Cut it in half crosswise (where a center thwart would attach).
2. Pinch down the cut tubes and glue them air tight.
3. Bring the cut sides of each tube together and sew or glue them together.
4. Sew or glue the now combined pinched ends of the two tubes together.
At this point it would look like a figure 8 if laying flat on the ground. However, what I'm thinking is that all four cut, pinched, and glued ends would be pointing skyward and the two raindrop shaped tubes would be right next to each other.
5. Take the thwarts and attach two of them crossways, a cooler's width apart from each other, between the two rain drops at the bottom.
You and a couple buddies could float down the river sitting on the thwarts and cooler with a bunch of useless rubber above your heads. Sound's kinda dumb now; I'll stop typing.