I would make a complete MDF box for behind the wall. And cut a hole the correct size in the metal ( no engineer needed as no chassis getting cut ). I would also make a MDF plate for inside on the back wall. I would mount the subs inside the cabin and screw through the inside wood, through the metal and then into the box. I would seal the inside wood and the box against the metal wall with sillicon.
You can cover the box anyway you want really.