When I tried this I used rubber tap washers (the ones I got actually had no hole in the middle so I could drill that to my own requirements). They are actually quite hard. It is not necessarily true that you need soft rubber mountings, the important bit is that their natural frequency is well away from the frequencies produced by the drive.

I have also mounted my PSU on foam rubber. This stops vibrations from the PSU fan from using the rest of the case as a sounding board, and also prevents any "knock" (which causes tinny rattles) between the case and the outside of the PSU. For this I used foam rubber self adhesive insulation strip, available from most hardware stores.