When it comes time to put the new cover on, use some contact adhesive around the edges of the seat pan and on the cover. Allows you to pull and stick the cover in place to the pan. This acts like a clamp to hold the cover while you then can rivet with ease. For rivet holes in the leather, I just use a grease pencil and mark the location of the holes on the pan inside of where the leather will be, then feel and punch the hole with an awl. Works pretty quick and saves any tedious layout work.
Loren
Baldwin Motorbike Saddles
www.MotorbikeSaddles.com
Baldwin City, KS