Kompo-Tech, Italy have produced an off-the-shelf slim (St.Ives type) seat for the Bonneville/Thruxton/Scrambler. It cost €226.56 (and they do ship worldwide i believe) and should be available later this month
Create a new classic look with this stylish narrow and flat seat. Fibreglass (GFK) base, comes with stainless steel brackets ready to install. Fits Bonneville, T100, Thruxton & Scrambler. Customize your saddle with the available options.
Comes in Black or Brown artificial leather.
Also available is the Oulton Park type Seat unit, for that cafe look and the Worcester Seat (another slim type). Check the Kompo-Tech web site for details.