I rigged up a great kayak setup for my 5 x 8 utility trailer. Thule makes a product called Top Tracks. The tracks are 5' rails that are actually designed to mount onto a camper shell roof. They happened to fit perfectly on the side rails of my trailer. Now I can haul anything I want in the trailer or can snap on my Tule crossbars & Thule 460 foot packs and have a kayak hauler. It's the best of both worlds.
Mariner Sails has the Top Tracks, 460 Feet, and crossbars of various sizes in stock for your inspection. There is also another version of Top Tracks that are made to mount on bare roofs via small shallow Moly bolt type fittings that go inside your headliner.