Suprised nobody has used expanding foam and baffling for contouring door cards for arm rests, did that on a old F350 a long time ago. Same principle as the guy that made a Custom exotic car out of a old Mercedes C230 in Germany.. Or was it Poland? I can't remember.. we fiber glassed over the foam, and SANDED, and sanded, and sanded, then..
You guessed it..
Sanded some more.. Then bondo'ed it and blocked it and then painted it all to look good..
Came out well.. Then my friend that owned the truck, bought a load of that MDF, and we scrapped the whole project.