There really are no fast growing palms that will stop at 20'. There are a number that would be great, but you would probably have to spend a lot and buy maturing palms and have a professional plant them. Most palms species make poor privacy screens, other than suckering ones, and your choices of suckering palms that grow that tall in your climate are extremely limited. Perhaps Accoelorrhaphe... Maybe a bunch of closely spaced Butias. A Hyphaene might work, though not sure how cold you get there... but it would be a pretty pricey palm, or you would have to wait about 15 years for it grow tall enough.