First, sponge is not a good idea if it covers the components on the drive itself. Current drives generate too much heat to be covered with anything. As for shock protection, unless you're driving an old beat up Camaro or Fiero or something, your car's shocks should be more than adequate to keep the drive from experiencing anything beyond its specifications.
I've mounted my system to the chassis of a '92 Mitsubishi Eclipse, no shock protection considered, and it did just fine. Survived a rear-ending at 45mph too!