Need ideas for interior trunk panelling.

Your idea of the thin particle board isn't a bad one. Attach the panels in a way that's not permanent: push pins, small screws, what have you. Then you can take 'em to the local upholstery shop to get the panels covered without having to leave the car. Once they're done, install them yourself.

Either that or find a good, reputable upholstery shop to start talking to. We have a guy here local who touches a lot of high dollar hot rods and antiques. He treats them with a lot of respect. Just what you need after some of the things you've put with on that car.