Update
In order to be able to stack disks outside computer cases, the solution i found was this
I got some of those grills used for paint, bent them so that they can hold the 3.5 inch hard drive through sc4rews, then i had a 12cm tube like aluminum tube into 1.5 inch pats, then glued the Sorbothane pads to the aluminum, then i duct taped the grills (that hold the disks) to the aluminum frame, and now i can stack them above each other.
Please be aware that vibration from disks affects other disks VERY considerably, i have had unbelievable failure rates with disks before this, the disk transfer rates also drops drastically when the vibration from other disks is high.
You might say that disk manufacturers claim that disks are relatively vibration resistant, if you dig deeper, you will find that vibration caused by a spinning disk is an exception to this, the type of vibration other disks will introduce.
Because i have many terabytes of files and backups, and because every PC i have has at least 4 hard drives attached to it, the vibration kills the performance of the hard drives, when you have 4 hard drives in a case, every hard drive is affected by the other three hard drives.
What i did was that i got a pack of vibration isolation pads, some metal grills that i use in suspending the disks (so that they can get better ventilation) (grills like the ones you could use in barbequing).
My hard drive failure rate went down considerably, actually, i have had no hard drives break down since i implemented this solution (I used to get plenty of failures).
So, the bottom line is, turns out the ONLY MATERIAL that isolates the vibration well enough (Actually so well) is called “Sorbothane Vibration Isolation”.
Anyway, here are SOME of the photos i took to demonstrate that.