I made "custom" packing to send a head out for work, using that void filler that comes from the hardware store in a spray can and expands. Put your parts in a plastic bag first, so no spray can touch the parts themselves. Lay an other sheet of plastic down and tuck it halfway down the sides, spray the foam in and close up the top to keep it inside and flat, do the same at the bottom. In your case, you prob want to protect the threads on the tank studs (and keep them from poking through the box) with some nuts, or better, use them to fasten a pc of plywood to the bottom that will make extra room around the perimeter.
Carpet padding is cheap and effective insulation, too. One main thing is to ensure that the parts cannot move around inside the carton.
If you have some 3/8 or 1/2 ply and a saw, you can make 6 custom pcs to line the container with. Your final package(s) shd be able to take a 4 foot drop without worry. I agree that the side covers shd go in a separate box.. Good Luck!