H18 wings fit with a few modifications. They install on the 18 in front of each beam; on the 20 they have to fit in between the beams, so the mounting "ear" on the bottom leg of the front wing post has to be turned around 180 degrees. A few washers and they fit very nicely. Need to cut a slot in the tramp to line the leg up with the front beam, and drill a hole in the beam for a long bolt. On the back beam, replace the forward "saddle" bolt with a longer one, and the wing fits right on! The support leg goes right down on top of the top beam bolt like it was designed for it! Little tough getting the longer bolts through the holes, and holding them tight while you bolt them up (I made a long flexible socket extension). The only really tricky part is the front support legs. I made up a piece of aluminum angle to mount the leg to the deck edge, and had to cut and move the support leg to hit this. The angle is bolted to the hull side.
Before you yell at me, I did this on a 20 that had been hit by a car; totalled by the insurance company and I bought it back. Figure the extensive repairs removed it from race status anyway. Also already had a (very tired) H18 Magnum that doanted its wings before being sold cheap. Been going strong for three years now as a play boat. Awesome! MUCH nicer to sail than the old 18, and carries weight better (wave smashing with 5 adults in 15+, for example). Kids love the wings, although you can't dip them as low as an 18. Even my 70+ mother can enjoy a sail in moderate winds. Single-handing off the wings is a thrill, too. Couple of pics here http://www.thebeachcats.com/modules.php ... php&page=2