Soft Coolers are a popular, non-face-bashing alternative that make good sense for any type of whitewater boat. The 21 Quart AIRE Trapezoid Cooler for float tubes, wider i.k's, and rafts comes in teal & gray two-tone, NOT the red shown.
This Cooler is made of inflatable kayak weight PVC and closed cell foam insulation, and holds it's temperature better than almost any other soft cooler we've tried. It has a carry handle on top, and two D-rings on each side to run cam straps through. These are not the beefiest D-rings by any means, but if you utilize all four of them it should secure the cooler well enough in a flip. Run a cam strap over the top and under the main lift handle for extra security, and then there is no way it will escape.