My trailer, which I picked up 3rd or 4th hand, was a small boat trailer that somebody customized. It has a wooden box for paddles & stuff, about 4x6x1feet, then the "trees" are added onto the ends of that. It's set up for 4 canoes. The trees are galvanized fence posts with 1" steel square extrusions for the top beams, and slightly modified 2x10s for the lower beams, with appropriate hardware for holding it all together. Not pretty, but definitely functional, after 30 years. Though I don't think I'd take it on a long, unpaved road trip.
Be aware that canoes aren't nearly as heavy as the motorboats for which the trailers are designed, so the leaf springs are a bit punchier than your canoes might like. I've heard that some people take a leaf out of the springs on either side to soften the ride, but have no experience with that.
