My grandson just had to have this fiberglas seat that I got from S&B Recumbents ... the bottom mount was sawed out of a Direct TV mounting bracket ... I kept a bit of the bend in the rear for rigidity. Welded to the frame ... there is no flex in it ... and it's wide enough to give good support to the seat bottom.
About halfway up the seat back ... look inside the purple rim ... and you'll see the bracket that came with the seat. I just welded it to the frame. The seat is a compromise angle that will meet the needs of the grandkids. Next to work on the steering.