I'm not sure this is a good idea. You're putting a harmonic (spinning platter) on a harmonic (spring feet) so you're effectively MULTIPLYING the vibration. That's going to damage your drives..
If you're getting really bad noise and vibration from your spinning drives, perhaps the DRIVES ARE DEFECTIVE. Or check the fan, too. Or if you're just particularly picky about noise, try putting some dense noise-dampening foam (the type that suppresses movement, not exaggerates it) under your Synology. But first, CHECK THE DRIVES.
EDIT: the egg-crate foam you used is designed to catch and trap sound waves in air. You need to use mechanical vibration damping foam.