Lol!
Well from what I've seen, people have mocked up a MDF template to run around the sub to act as a strengthening mounting plate.
You could I guess put the MDF mount behind the metal enclosure, but might be very tricky
If it was me, I'd but the surround on the outside, spray it black, mount the sub on top, then cut around the plastic to allow the sub to poke out, but keeping the plastic to cover the surrounding metal area.