I have a 12'6" self bailing raft with oar frame and oars. It would be similar in weight and dimension to your pontoon. I have a set of bike wheels, 20" kids mountain bike wheels to be exact, mounted on a piece of steel tubing that is just a bit wider than the raft. This tubing is actually 'sleeved', so when not in use you can collapse the shaft down. Fully extended it's over 6' from wheel to wheel to accommodate the raft, when collapsed its half the distance. There are holes drilled through both pieces of tubing that allow you to put clips through to lock them out. Take out the clips and collapse it back down. There are a couple of rings welded next to each wheel. I can lift the boat up and slide the axel under the boat, preferably when it's still floating in the river, then use a tie down strap on each side to lash the assembly to the midway point of the boat. I happen to have D rings at the mid point of the boat so this is very easy. Once the axel is tightly secured to the raft it is very easy to push the boat over almost any terrain with one person. You can also leave most of the gear in the boat as the wheels and axel are strong enough to carry a bit of weight.
It's a very basic axel with a couple of bike tires on it, cheap to build, and as long as the diameter of the bike tires are big enough, very easy to maneuver.
Kinda sounds like what you are looking for.
Sorry but no pictures of it.
Kevin