I used 1/8" thick wood paneling, used for house interior doors on my Model A and it worked well. It is light weight, somewhat flexible and also strong. It will not disfigure like bristol board if it happens to get wet. It is great for custom panels as you can easily permently attach items like your arm rest. The only negative I found was that it was not as flexible as thick bristol board, so if you have a lot of curved surfaces, you may have to use the bristol board. Good Luck.
Rob
I love the smell of burning rubber in the morning!