depends - how big are you? or rather - how big is your ass?
most of the generic cafe seats I see run from 14" to 18" for the seating area and 27-30" in overall length. There is one on ebay right now for an xs650 and it has 15" of seating area with 14" of area for padding, and 30 inches total length.
back when JT was at confederate, he used to make molds of his ass and then built the saddle to fit so it would be comfortable. I belive he build custom saddles for each of his customers this way. If you are going to build your own seat - I would suggest you go this route since then it would truely be unique.
with bum stop seats, the seat height gets lower which means your leg angle changes to something more accomidated by rearsets (unless you are short).
and as for molding your ass....ass sized tub from walmart or k mart and a lot of plaster. the plastic bag is a nice touch if you like your clothes and yeah, pretty much just have a seat. Once you have your mode you can make a positive of your ass to use as a mockup in making your seat (ie shaping your foam). You think I am kidding but really it is only a little funny.