Popular seats are Mazda Miata and Porsche 914 seats. It really all depends on how tall you are and your body size and shape. The cockpit of the Concept 1 looks cramped so you want a seat with a thin seat back so it doesn't push you into the steering wheel and a thin bottom so you don't end up looking over the top of the windshield. Good luck.

Also 19" wide is about the limit on width to fit between the tunnel and the buggy sides.

Mike T