i was in the same boat. so my pops and I made some. we took a square bar, welded a loop on the top, then welded a flat bar on the bottom, drilled holes in it just past the width of my thule bar, then set up a bottome plate, drilled holes in it then used some long bolts, washers, and self locking nuts...kinda' ghetto...you can't really just yard on it when your crankin down yo' boats, but it'll still hold a full que of boats.
you could probably make it stronger if you reinforced the way you attach it to the thule bar...but mine seems to work just fine. hope that helps.