The Lyman 36 is a good option. I have one on my 1903 also and just sold one for the later model 1952 to a guy on the forum. Another cheaper option would be the Williams sight where the aperture slides on a ramp for elevation. John Barsness has a couple of photos of his 1903 with one of those on it in the May 2015 issue of the American Rifleman magazine.