Sure. Anything that will help take up volume inside the box would work. 2x4's are good because you can easily find out how much volume they consist of.
yep 2 x4 's scrap mdf. Just make sure you attach what ever you add once you get the ideal volume so it doesn't rattle around inside. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif