There's some stuff called Asaka Pax-Mate that is available at
SVC that is more of a flat sheet than a foam, but it does very well.
A cheaper alternative of that type of stuff, directed toward the automotive market, but just as effective on case panels, can be found at
Parts Express
BTW, open-cell eggcrate-style "mattress" foam will do you no good, as the open cells do not absorb any energy/sound. You need the closed-cell stuff and, in the kind of thickness you need for it to be effective (say 2") it can cramp your style on thie inside of a case pretty seriously.