Rob have you thought about fiberglass instead of duct tape i know it's too cold for it now but what do you think?
i thing im going to do a mess of it, im going to make door panels for the rear seats out of 1/2 MDf or Ply and fiber those over and carpet, and then cut MDF to fit between the box and doors tight, buy a back deck pannel skin it and fiberglass it, then mdf the box right into the car and carpet it right in.