I have seen those, but haven't actually used one (haven't even used a pontoon...YET. Boat should be here in another week or two). There is a cheaper idea for anyone that can weld or knows someone who can. It consists of a sleeve (about 4-6" long) that a bicycle fork can fit in. All you would need is an old bicycle fork, the tire size of your choice, and a pin. Weld the sleeve to your frame (in center toward the back preferrably). Drill a hole through either side of the sleve and do the same on the top post of the forks. Slide it into the sleeve and stick the pin in. I'm not sure what the actual name of this type of pin is, but it has got a u-shaped clip on it that keeps the pin from sliding out. The only down side I can see is that you would need to remove it before you get in the water (unless you weren't worried about drag). If you really wanted to get detailed, you could rig the sleeve so that it releases to allow the tire to fold up out of the water.