Your bike looks great and I admire your creativity. I agree I wasn't a fan of the banana seat but I simply can't put seat cowls on my bikes because I usually have a passenger or a backpack that would scratch the hell out of it. So what I've done is on my F3 I took a piece of red vinyl and sown a rear cover, new strap, then stretched and stapled it to the seat.
For my 929 I just made a full rear seat cover out of black and yellow vinyl to follow the graphics of the bikes. This way someone (ie the girlfriend) can just right on back and not have to worry about removing or replacing the seat cowl.
