Originally Posted by
bryson240
he made a fiberglass box and it was like rattley sounding then he made a mdf box and it was clean
A fiberglass box shouldn't sound "rattley" at all. The fiberglass makes the box a one, super-solid, airtight piece.
pour the fiberglass and "paint" it all on the inside of the box, this will make it even more solid and airtight. Then you can paint the outside of it just fine.