amp displacement volume, psw 1200

To eliminate rattles and increase longevity, I am replacing the amp section in my psw-1200 with a piece of .75" MDF, and I am wondering how much volume the amp is calculated to occupy in the subwoofer cabinet. Should I glue blocks of MDF onto the replacement panel to replace the removed volume, and if so, what is the volume I should use (I'm not about to play Archimedes, if you know what I mean)? Or, is the volume inconsequential?