Just so you have an option. I used a bench seat out of a Jeep for my Series. Fits perfectly. But I don't know if you would want to make your own mount to lift it off the floor a bit. It sits way too low for an adult. But they are cheap and easy to find on Craig's List.
Project: Aux 110
1965 IIa SWB
1988 Saudi 110 (soon to be 300tdi)