I would think a cooler frame/trailer frame would be a good bet. See the link below for example. You can rig the cooler at different spots if it has its own frame. If it is in back, the oarsman will end up opening the cooler and passing the beers. Place it mid raft or up front, that way the paddlers get a beer FOR YOU!
Lots of flex in the back of a raft. Extending a stern frame may have pluses and minuses depending on your river run.
Here the link for example of cooler frame