An Era-300 on a bookshelf?
That really doesn’t sound like a good idea given the multiple drivers and their locations. You might want to re-look the owners manual section on positioning.
For a bookshelf a Five would be far better.
I’vee had had no vibration issues with my Play 5 Gen 2 or gen 1, or my Era 300s that are sitting on an open shelf.
If you do have issues and want more damping simply get a soft foam kneeling pad and stick under them. You can put a sheet of something stiff on top of it if you want them to not sink in a bit. Wood if feeling fancy or just a kitchen cutting board. You could even trim both to fit your shelf or the speaker’s footprint.
Soft foam pad:
https://www.amazon.com/gp/product/B077XSFQZW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
If the wall behind the bookshelf is shared with the neighbour, you may want to reconsider the location of the 300s. This would be bad location because bass can travel through walls very easily.
If you mostly listen to bass heavy music, be a considerate neighbour and place the speakers away from that wall.