You could just go plain HUGE, and get a set of Aire Lion tubes( 25.5")?
I've often thought a set of Royal Flush Tubes would be Pretty sweet in a boat that size, Jacks Plastic stubes the ends, gives it some rocker. Sounds like you end up with sweet handling, and ability to get it down tight, low water stretches if your relatively light, then if you want to load it down for a bigger river, you have reserve boyancy from the rocker, plus the tubes are 25", and the blunt ends make it cary like a bigger boat.
Of course listening to your intended purpose probly makes sense, just get excited, new Cataraft tubes!!!!