Cheap, thin ply wood (AKA interior paneling) works great. Wrap it with indoor/outdoor carpeting in your color choice and secure it with trim screws. The space inside the door is filled with foil-faced rigid styrofoam insulation for additional sound and thermal insulation.
I agree, the metal option seems to be the most rugged, but the I/O carpet really makes it nice, and is easily cleaned, even if submerged.
I used fiberglass sheeting, as used for industrial kitchens and bathrooms. It's a bit pricy, but very durable, easy to clean and since it's white, makes the interior extremely bright, which I like in my 110 regular which has no side windows... I still used the insulation under it, and the truck is very comfortable year 'round.
Owner: James Leach Global Expedition Services.
1995 110 Regular