My '27 RPU is original Henry Ford steel, but the body is the same size as a T-Bucket.
I cut the stock seat frame down so I would sit lower in the car and then made a plywood seat base, cut holes in it, and used webbing to cover the holes. The upholstery shop then covered the seat base with high density foam.
For me, the uncomfortable part is the lack of leg room. Because you can't stretch out your legs you put most of your weight on your tail bone, that's what makes the ride uncomfortable, not so much seat related as the length of the bucket itself. I cut down on the amount of padding in the back seat for that very reason.
I think Ford figured that out. Model A Roadster's have a longer leg room, which makes a huge comfort difference for longer drives. And that's also why some of the glass body makers started making the bodies longer.... but it pretty much messes up the visual proportion of a Model T, IMO.