Model Car Garage sells a set of thin etched metal saws that are so small they will cut the scribed line on the door and sometimes leave material. A hot knife is not an option in my opinion. You know what happens to the plastic when you apply heat, it warps. Hobby Link Japan also sells etched metal saws that are slightly thicker then MCG's. Even if you accidentally take too much off when cutting the door out a stirp of .010 Evergreen will fill the void.
I use thin styrene sheet (.010 or .015 ) to cover the truck bed. I add strips of styrene ( .030 ) around the edge of the cover and round the outside edge, this locks the cover in place when finished. Sometimes I must use a thinner or no piece at the front edge of the bed because of the closeness of the cab. To finish you can cover with thin leather or fabric or you can shoot the cover with Faux Fabrix or something similar then finish with SEM Vinyl spray. It looks just like a naugahyde cover. Feel free to contact me if you have any questions.