I would not re-invent seat springs, look at a few bench seats. Consider your suspension/ride/roads relative to how seat springs and foam evolved.
Foam over wood might solve a problem with the small size of these cabs. I think you'd find it rode hard on longer trips.
I'd probably start with a 80-90s bench spring set , or even two so you could cut to shape for the old frame and join the sections with hog rings
next you need a thick layer of fabric to stop the springs from cutting into the foam over time, Jute, burlap or some carpet with a woven backing.
then foam of choice shaped as desired