I used 17" plastic wheels, 1/2"x2'x2' birch plywood, some 1" PVC with caps, and machine bolt for axles. The ply was cut into 3 pieces and slotted. The heads of the bolts were ground off and holes for cotter pins drilled through the shafts. These were then epoxied into a wood channel. The PVC was cut to length, slotted to slip over the 1/2" ply and secured with screws. Once the pipe was capped, split pipe insulation was glued onto the pipe.
The entire assembly can be broken down readily for ease of storage. The "lightening" holes really serve to strap the kayak to the dolly and they add a bit of style. After a good sanding I'll put a few coats of polyurethane on the works. I'm very happy with the results. It is very serviceable.