Seal around the edges with spray foam such as "Great Stuff".
Cut a sheet of sheet rubber such as rubber roofing material slightly larger than the box cover so when the cover is screwed on the box is sealed off from the room.
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.