Usually re-use originals, but they can be made w/out too much trouble. I would use something more stiff than cardboard.
Good topic. My '67 headliner was replaced once back in '79 and fitment since was always a bit saggy or wrinkled meeting up to the package tray and tops of the quarter trim shortly after it was "Professionally Done". Likewise, there were none of these "something more stiff than cardboard" pieces found when I removed that old headliner.
It sounds like we Coupe owners could all use some guidance on how best to duplicate something we have no original examples available to pattern a replacement from. My memory from working in a salvage yard +30 years ago, is trying to visualize the material (heavy cardboard is close, but certainly not correct) so that I can obtain a similar product from a hardware store or the likes. I am thinking it was very similar to the cardboard backing of an original door panel, am I getting close? (not hardboard or particle board)