You're not talking about expanded polystyrene, are you? Made up of lots of little balls?

As an alternative, try expanding foam. Cover the item up REALLY well with cardboard and plastic so that none of the foam can get in, position it with more cardboard exactly where you want it, then squirt the foam in and it'll expand to fill the holes. Leave it to dry, and you'll have a nice form-fitting shape in closed-cell foam left behind. But you MUST make sure the item is completely sealed in, cos if the foam gets in then it'll fill up the instrument as well, and it's almost impossible to shift once it's dried - you can cut it apart, but you'll never get the last bits off.

Graham.