I used to put a thick chunk of foam rubber in there..it needs to touch the beater area and the front of the shell...you don't need to fill the whole pad. it should stop the shell resonating.
just remove the black trim screws..then the shell screws and remove carefully so as not to crack the shell. the trim screws are black dome headed and the inner screws are often silver flat headed countersunk screws. a lump of foam around 5" thick should do it [I'm guessing here though so measure it ] .
It needs to be oversized a little, maybe an inch or so , so it is compressed / squeezed when you replace the shell .
good luck,
bruce