You will need a solo seat hinge, a seat pan, a set of spring mounts, seat springs, and a solo seat. Check out the How To Mount A Solo Seat
article to see some examples of parts used and how to mount a custom solo seat. The solo seat hinge mounts to the front portion of the seat and to the frame backbone. This hinge allows you to raise and lower the seat so it floats on the solo seat springs. The spring mounts are typically welded to the rear of the frame, and they hold the springs that the seat rides on.