Seat springs are produced by Snyders. I see for the 28-29 Pickup they now have a "seat" which is 2" shorter - and a corresponding "seat back" which is extended 2" to match the lower cushion and still get caught by the "catches."
I went with the original bottom but made up a seat back which was about 2" thinner and gives a tall person (I'm 6-3) some more room to put their feet on the pedals.
I used "sear-o-foam" (i.e. white cushion foam formerly bought at Sears-Roebuck) but there are today other sources for cushion foam. The back behind the foam is plywood and the covering is brought around the edges and stapled to the back.
Joe K