I'd just go for the removable crossbars that Rich posted above. I got a pair, and they are seriously great. Since they're the same square bars as Thule (in fact, they're made by Thule) all the accessories fit perfectly. I bought a fairing for the front, and got a set of Thule lock cores to make sure nobody steals them off my roof. They go on in 5 minutes and I've hauled a ton of stuff with them. Most recently I strapped on the biggest cargo box Thule makes, filled it with about 200 pounds of stuff and drove the family from Ohio to California. When we got there, I had a 10' 5" surfboard strapped up every day (we went on a surf trip down to Newport and had 6 boards up there at one point) and it stayed solid as a rock.
What I like the most about them, though, is that I can take them off and my wagon looks sleek and streamlined again!