The original had a spring support for the cushion. The modern, easy way out is to have a futon mattress made to the appropriate size, and support it with wooden slats running from front to back.
Plans include detailed drawings printed on two 24″ X 36″ sheets and a bill of materials.
(International customers please E-mail before purchase for shipping estimate)
$17.49 + $2.45 Shipping to US addresses