my kayak dolly.

Exhibita pretty much nailed it. I went the homemade route briefly, and broke several incarnations of carts. I got a primex cart (now wheeleeze) and it was the best $ I've spent since buying my first kayak. The only homemade carts that really work are too heavy and/or do not break down for stowage which is a necessity for me.....

I second that.

  I only broke two homemade carts before I gave in and bought the Wheeleez, so I don't know if I would have hit on a good design if I'd tried a half-dozen more times.

   I tie it to the yak just the way the Wheeleez site shows, and if you get the straps tight and put the cart under the yak seat area, it stays put.   
     Don't let the straps cross as you're looping them over the yak or you'll pull them tight only to realize they were just binding on each other until the first big bump, when they loosen.

My creat cart still going.
http://www.norcalkayakanglers.com/index.php/topic,16164.msg161149.html#msg161149
The only issue is ground clearance.