I was asked to make a quarter tone C trumpet. What I decided to do was use a rotary valve for the 4th, quarter tone, and place it immediately forward of the 3rd valve. Other people have done this by inserting a rotor in the main slide, but I felt that this makes the trumpet front heavy. Before clean up and tidying up the lever assembly, here it is:
I am very happy with the way it plays, but the quarter tones drive my ear nuts.