Someone a while back said before you go off designing a kayak trailer you ought to check out Harbor Freight. They mentioned a 4'x8' folding trailer, but I liked their little boat trailer.

I purchased two chain link fence posts from HD along with some galvanized rails for the arms. I welded all the joints after bolting up the DIY kit, then removed the bolts. Made custom cradles out of treated 2"x6"s and lined with 5/8" minicell foam then carpet. It will carry 4 kayaks or 2 kayaks and a canoe. (the link has a picture at the bottom of it loaded with 2 yaks and a pirogue)

4' wide at the wheels, I took out the small leaf in the 2 leaf spring setup so it would not be too stiff, seems to work great so far. The length works fine with my 17' cirrus.

Thanks for the tip on Harbor Freight D.I.Y. trailers to this board.