Did some more test flying, then fitted profile bits with urethane bond, weighted with bricks and screws.
After a break note the lumbar support had slipped, and chiselled it off! Made another from some hardware store extruded polystyrene and reglued.
Made a narrow base for the headrest to sit on, intending to locate it with some hinge, so it can fold forward for rear seat entry. Stuck some 1/16" 2024 on both side of the hinge point.