Hi,
There is no need to put anything in the M80 In-Cabinet cavity. Simply make sure that all the surfaces of the speaker enclosure (except the bottom one) don't touch your false wall structure. Will you be using the EP500 In-Cabinet subwoofer model as well? I don't think that the sound waves from the speaker will seriously vibrate your screen. Howver, it might shake and/or move your projector it it is resting on a shelf. I had to lower the volume on my subwoofers because the projector would jump on its shelf. I have a friend who had to put "blue stick on" on the legs of his projector because it would slide on the shelf. Very powerful these Axiom subwoofers!
