How thick are the seat back cushions?

You really don't need much padding here, but you do need a lumbar support and an angled seat back.

A 2" thick upholstered firm foam pad will do it.
Upholstery shops carry foams in different thicknesses and densities.

Making your own seat & back bases from plywood is easily done.
And upholstery shops will have no problems padding and upholstering the plywood backs.