GRP replica front door panels have been made. The problem was that the plastic is very thin. The millboard is thicker, it is true, but there seemed to be some issue in making the GRP option stay put.
Millboard is a product from paper milling. It is a dense board containing a lot of rag waste. This gives it a characteristic grey colour and a sort of laminated effect if torn. The rag content makes it tough and flexible along its length. It can be coerced into angles to fit in restricted areas which a pure board based product would fail into a fold. It is still used in vehicle upholstery but usually unsurfaced with sound deadening materials or such bonded to it. The standard surface is, err, unappealing. It is used as the base for Messeschmitt interior panels in most instances, for instance. So it is offered with an applied coating, which can be coloured but more often black or grey on one or both sides. This contains clay and is polished and, I think, covered in UV varnish. Thus coated board needs more care than uncoated board when bending to fit. The fuel tank cover is a test of Millboard quality as cheaper board will fail at that angle over distance. It will not cover a compound curve unless moulded to do so. This, and bending to shape, is done using gentle steam and pressure. The surface has a pretty high scuff resistance however.
Something similar is fitted to the boot floors of NSU Prinz, for instance. Trust this helps identify the material in question.