Go here :- http://archive.org/search.php?query=edgar%20k.%20atkins

Download all the free bike plans.

Look through them and one shows how to build a 2 part seat so the base is similar to Brads piece of plywood and foam etc and the back rest is tubing and mesh.

Couple thoughts on seats ?

Really really need a mesh back to stop a sweaty back
Your bum can be on plywood/foam/vinyl it won't cause it to sweat
I doubt if the lumber bends in seat frame as you picture really work ? as the mesh will distort under your weight and lose any shape given by the seat rails ?
in the plans the seat back is shown with bends in it , it could just as easily be made using 45' mitres for the 90' bends so no bender needed.
The base could also work made from one of those big bum seats found on some bikes and exercise bikes so you would only need to build the back rest.
https://www.ebay.co.uk/itm/WIDE-BIG-BUM-BIKE-BICYCLE-GEL-CRUISER-EXTRA-COMFORT-SPORTY-SOFT-PAD-SADDLE-SEAT/232040388965?epid=1083246654&hash=item3606af5965:g :XL4AAOSw8gVX7gMt


The back rest only needs mesh across the shoulders to work
The mesh could actually be nylon strapping as used on the Kingcycle seats ?[ early recumbent ]