One idea I've heard of is to use metal 2x4 framing filled with rigid insulation for the core. Then you layer thin strips of wood on the outer edges. After that you put 1/4 inch plywood on both sides, then paint it to look like wood planks. This makes a very light door with better insulating qualities than a solid door. I would use a knob in the middle and levers similar to ones from a manusl garage door opener.
