You could go to the lumber yard and get yourself some of that thin wooden panelling, then make a template using some form of cardboard, or maybe the paper you use for taping a vehicle off to paint it. Then you could just transfer the pattern onto that wood and cut with a jigsaw. Or perhaps you could use some kind of plastic sheet if you can find some. Just some thoughts. You might try a scrap yard for the headliner just for the board itself.
ORC Land Use columnist
My November article on ORC