I believe from an originality standpoint that you just use the regular rear trim panel suitable for your car based on color, piping, material and any applicable grain and embossing pattern. There was an optional rear seat kit for the TR4/4A that could fitted to the TR5/250/6 even though it was officially never listed for those cars, but IIRC, it was a seat cushion only for the rear shelf that was removable and held in place with little ball studs that snapped into a clip. As far as I know, there was no seat squab for the rear on the TR4/4A seat like there had been on the TR2/3/3A/3B. I guess that you could always fabricate or have a trim shop fabricate a rear squab and use a similar sort of ball stud and receiving clip arrangement for mounting purposes.