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sound deadening. pick it up from auto barn or anywhere that sells sound stuff.
even if you go the MDF box, put some in there. This will stop sound from escaping out the box for everyone outside to hear. also stop vibrations.

also another tip, MDF weathers. if you use it clear gloss it and paint it or cover it in something else.
perhaps, make the box for the sub out of MDF, then sound deadening on the inside and out, then a metal cover over that.
this mite be a bit of overkill, but the less the cops on the outside can hear your music, the less interest they have in you.