Why would carrying cargo in the rear with seats down be illegal? I highly doubt it would be, unless it restricts view from all of your mirrors or something similar. If its toolboxes which arent too big, these would be fine.

As you said go to Clark Rubber and get them to cut the shape out for you, be a pretty simple process. Cant really think of another/better way of doing it, you have hit the nail on the head there!