Tip for filling in the slots on the chassis:
Instead of using plastic filler and/or rods, use Magic-Sculpt, or some other similar two part epoxy putty. Just squeeze the Magic-Sculpt into the cavities and, using a wet finger, smooth the epoxy down to match the contours. The putty doesn't shrink when dry and there isn't the possibility of warpage as there is when using solvents to bond the styrene rods. Once the epoxy is dry, you'll only need to use a medium sanding stick/ paper for final shaping.