The only rear cassette that it doesn't rub on is the large 32 tooth cassette. It gets progressively worse as I go down the cassette. I've got an OLD Shimano 28 tooth chainring laying around that I might try next. It's steel and doesn't have all that fancy machining on it. I'll let you know how it goes.

As far as washers go. I think all I need is one thin washer for each hole. It wouldn't take much to make it work. My worry was will the chainring be strong enough. I normally only use my granny gear with the large back cogs. As soon as I hear any rubbing I shift up to the next chainring. But, I want the extra capacity for those long climbs.