Panel base is probably 1/8"polyethylene sheet since it is bent on the top. Permanently bend to the curve with a heat gun. If a panel is pretty flat you can use 1/8" Masonite & Luan plywood. The arm rest can be made of anything you want - easiest way is use wood (2 x 4s, plywood, etc.) and cut, glue, grind and sand to shape. Cover it separate from the rest of the panel and glue and screw it to the door from the back side once the rest of the upholstery is on. If you use enough clips described below attaching the panel to the door, you don't need to anchor the pull to the door. If you wanted more security , you could use a finger bucket from a junk yard car and screw it to a bracket attached to the door, which is what it looks like this guy did. The various different material sub panels on the door are usually more 1/8" panel material cut to shape, covered separately and glued to the base panel. The panels are covered with 1/8" or 1/4" closed cell foam rubber before applying the upholstery material for smooth and pleasant feeling texture. Attaching the panels is super simple. Just align the bare base panel to the door before covering it and drill a series of 1/4" holes through the door and panel @ ~6" spacing. Snap one of these clips
into each hole and the panel pops off with a screwdriver and reinstalls with a shove of the hand on each clip.