if you're not worried about it being original, (and maybe looking some top heavy), you might try a seat from an exerciser. usually one of those things that looks like a cross bettween you rowing a boat and riding a horse (i don't know what they're called)
this one is probaly for certain wider than you want (i went with the widest i could find with the big tire), but i've seen them in sizes bettween this and normal older banana seats. you may have to get imaginative with the mounting and rear support/sissy bar, but if you can find one cheap and can work it out, you may get the comfort you want.