I'm trying out re-curving the fenders to better fit the 20" wheels. This involves heating the fenders in the oven, or with a heat gun, and then forming them around the 20" tire. The temp will only be high enough to soften the plastic, and the tire can be protected by placing a used towel, heavy rag between it and the fender while forming.
Should be able to do this at 200*F or less. I've been very busy, so haven't been able to try this out yet. I'll post when I know for sure.