I built the seats by taking a pair of old Miata seats that I had laying around, and cutting the top layer of foam off of them. I traced the silhouette of the seats onto 1/4” plywood and stapled the foam and the fabric to the plywood. I believe that I posted pics earlier, but I have them out of the car and have redied the fabric, so I’ll take a detail photo and post it tomorrow.
I just Velcro them to the floor and rear bulkhead. They provide me with lateral support and about 90% of the comfort of the full Miata seat, and maximize my headroom.