@nic919 As you are now looking like you are to attempt to Install a Wall Shelf as a Mounting Platform, it is worthwhile considering the Shelf is best thought of as mount for a Isolation Footer > Sub Plinth > Isolation Footer > TT.
If the Wall Mounted Shelve can be exchanged for a improved material, it can be thought of the Structure as having Two x Sub Plinths under the TT.
I recommend Densified Wood today as a Plinth or Sub Plinth Material.
I have trialed and encountered through others, many materials used as a Plinth and Sub Plinth. In my home set up and as a result of loaning the Densified Wood I have, it is discovered to be a better material than a highly compressed foam I have used for many years as a Material that was a betterment to many others.
The Densified Wood is reasonably easy to source.
A wall Mounted Shelf, if mounted in a way that is totally trusted for its secure attachment, can serve a dual purpose and one that is not too commonly seen.
As your TT Set Up is relatively light weight, the shelve structure can also be used as an Anchor Point to suspend a Platform/Sub Plinth from and used to mount the TT on the Suspended Platform/Platforms.
As you have spoke about a DIY Shelve, I would suggest a seriously consideration of this alternative idea of using Suspension and try it out.
There is a lot to be gained from the use of a Suspended Platform. The method is able to offer a improved method of isolation beyond the coupling to a Solid Shelve as a method of Isolation, there are other alternatives where thousands of $$$$'s can be spent to get a betterment.