Fiddled with front seats for a while. Seeing the shoulder supports don't actually support my shoulders, consider what structures/arrangements would. Played with some bits of wing foam for lumbar support until the thigh support collapsed under me.
Hot glued the untrimmed passenger thigh supports to a pilot one, and routed them to the same shape.
Made spacers from 75 x 35mm scrap timber for installation of the thigh support bulkheads, and hot glued them all together, then hot glued passenger thigh support to the bulkheads.
Rather than the micro I had used before, fitted strips of 1/4" last-a-foam inside bending grooves in the top of the passenger thigh support with urethane glue