I agree with Damocles. Taller windshields are very subjective. Take two riders the same height with the same windshield and one will hate it and the other will love it.
The National Cycle does seem to be a generic plexiglass windshield that I've never heard espoused as a good alternative for the KLR. I say either start hacking it up incrementally to try to suit it to your liking or just ditch it. I doubt anybody would be interested in buying it.
I get the smoothest ride, in my opinion, with no windshield at all, not even the stock one. I must say though, a tall windshield, despite the annoying buffeting with the one I tried, is nice to have when it's raining really hard or it's really cold out.
I'm keeping my Madstad Adventure windshield, but have never put it back on since I took it off.