Inside... It will act some what similar to polyfill and your sub will think the box is really bigger than it is. Dynamats main job is to slow down sound waves, and thats about all it would do inside the box. If you are worried about leaking air use fiberglass resin on the inside which will seal everything up perfect, PLUS you get a nice little high.
Outside... If you are worried about bracing add bracing...Either double baffle it or stick something inside to brace it. If you are worred about vibrations, dont, dynamat wont fix it enough to notice it.