I have had some success with a model I saw some whitewater boaters using last summer. It is simply a pair of skateboard trucks with wheels and a piece that attaches to the hull. There are some fairly large 6" foam filled off road skateboarding wheels available that I think would work well. The trick is the piece that mounts the trucks onto the boat. I have experimented with sveral designs and think that a piece done in plastic that webbing straps could fit through would work well. the wheels run $80 for 4 and trucks about $13.
That's as far as I've gotten.