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.