Instead of fabricating a seat or seats into the cab I came across a fairly decent (fairly rough) seat. I had two other donor seats so I replaced springs and wires, reinforced, and freed up all kinds of stuff. I had to cut and weld the seat ends on each side where the back rests.
The bottom of the seat had rubber vent-like pieces. I found some neoprene rubber in 6" x 6" on ebay that seem to do the trick!
I'm replacing some top springs and my cover should be in within this week.