I know the ones you're talking about and being a frugal type made my own. A couple of 8 inch tires and 5/8 round bar, with washers and presto pins from Princes Auto. A 2 x 4 a bit wider than the tinny and you're off to the race. I ran a 1/2 inch wide by 1/2 inch deep groove along the flat side of the 2x 4 to keep the bar from rolling away. Slide the contraption under the boat, attach with a ratchet strap over the top and go. I used mine on a 14' tinny with 7.5 Honda on the back, it was an easy pull. I would suggest not to use skinny tires, they dig into the ground and mud too easy. Tires something like these.
Tires