Here's what I did - cost me $66 including $40 for the donor seat.
So, I finally have come to a close-to-final shape of my new seat. I took a stock 550F seat, cut a bunch of foam out of the seating area, and then added foam to the rear of the seat. The donor foam came from another seat - a crappy old rotten 750 seat.
You can see the original profile of the rear by looking for the dark line between the layers of foam. The dark line was the top of the 750's seat foam, stained from age. Nice smooth surface however for the adhesive.
I've added bits of foam here and there as I shaved down too much in places - just stuck more foam on and then shaved down again. Will fine tune it to be perfectly smooth.
I've used a utility knife to do the rough cutting, and then I dug out a tool from my chest that I've never used. Not sure if it's a plaster tool or what - some form of rasp? Anyhow - it worked perfectly for shaping the foam to a smooth finish.
I'm also adding a layer of high density foam (grey) which is from those interlocking floor mats sold at Home Depot for your workshop or the multicoloured ones sold for kid's to play on. Then a thin layer from an old yoga mat. I don't have a cheap/free thick yoga mat to cut from, but this setup seems to be decent. Haven't sat on it yet, so who knows.
I'll get a pic of it on the bike later. So far it has cost me nothing to build, aside from the purchase of the 550F seat for $40. Buying black vinyl today ;D I haven't found a definitive souce of info on upholstery - not sure if I need a layer of fabric or anything under the vinyl. I think some vinyl has a backing on it - will be looking for that. Need to start working on a pattern for the vinyl - will do that with poster paper.