Have been working on the Seat pan. Originally I was going to RP the pan, but after a bit of experimentation I decided that it wasn’t strong enough to take a serious amount of abuse from my buttocks. Instead I cut and shaped come 2mm aluminium then redesigned the holder to fix onto the plate. Used PETG 80 infill in 3D print… feels solid. Have bolted frame brackets out of aluminium for now, but I 3mm mild steel versions that are ready to be welded into position once I deconstruct the frame for a final prep and paint.
With the seat cushion placed on top of this plate it would look a bit like the images below: