Today this was tested with a 100lbs person sitting in the back while I lugged it around. It held up quite nicely. The only thing I might change is the space beteen the tires and the kayak. If she tilted a little bit the wheels would rub a little bit.
Step 1: Materials and Tools
Now, every kayak will be different. So your sizes will change depending on your kayak. Heres a rough list of what you'll need.
2- 12" cuts of 2" PVC piping.-I used the thick walled gray stuff
2- 25" cuts of 1/2" PVC
2- Lawn Mower Tires Or Whatever You Have
12" of 90 degree bend metal plus some for later. (See Picture 2)
2- 3" Bolts and nuts to go with it
The length of my 2" PVC is based off the distance of my scupper holes. For sit-in kayaks, just make sure its at least 3/4 of your kayaks width or more, and the 1/2" might have to be longer than 25" so you have enough ground clearance.
Drill-For Putting Holes In PVC and In Metal
Saw capable of cutting through PVC (ie; Bow Saw, Hack Saw, etc.)
Welder-To weld a wall on the angled metal so your wheels can attach, but there can be many alternatives to the "Axle" design that don't involve a welder
Grinder-To cut out square to be welded on
Now I use the angled metal and such for strength. You could very easily just take a long axel and string it through to your wheels. It would make things a bit easier, but I'm a complicated man.