Hex,
I had a guy and gal pull up next to me at a launch. They made their kayak trailer out of a small boat trailer. They had a box on the main frame with doors that folded down on each side. They stored their broken down paddles, fishing rods, PFD's, tackle boxes, etc. Then they mounted a tower (like this ? ) on top of the box that would carry 4 kayaks. Sweet rig.
I think a single jetski trailer would be just about all you'd need to start. Depending on the length of the kayaks though you may have to extend the trailer's tongue a little. Not sure about that though as I haven't looked at them that close .... yet.
"Only the half-mad are wholly alive." ~~ Edward Abbey