The "zig-zag" springs should be available from the upholstery shop industry. Then you'll need some rough burlap to lay over them. Then some thick wool (?) and the various densities of foam rubber on top of that. THEN the covers. The foam rubber items need to be "automotive grade" rather than otherwise, if there is such a thing, for better longevity in heat, cold, humid, and dry conditions. NOT the same environment as inside a reasonably-temperature controlled residence.
Just some thoughts,
CBODY67