I found my old 305 x20 tire in the storage unit, while we are working on fenders and filler panels Sunday, I'll see if I can figure out a way to get a rear fender that will match the fronts, after looking at what everybody else is doing, metricks seems to be the closest to what you want, but I would like to see a hidden mounting plate if possible, but that maybe why their fender does not come down as tight to the rear tire because of the framework to mount it with.
I do not believe the radius of the original fender is going to work with the 20 inch tire and rim