Hello guillochon,
Firstly, let me welcome you to the community and to our Shake family!
Regarding your query, if you haven’t already seen them, there is a thread with some interesting suggestions here: Location of vault
And, also, always if you haven’t already seen them, these articles on our blog:
Securing cables and putting the Shake on a concrete floor should be enough to minimise the noise to acceptable levels (there will always be some localized noise, but it will be reduced to sporadic episodes and low levels, unless you live in a very ‘noisy’ location).
Consider this as an example, I have my shake on the lowest shelf of a bookcase, on a wooden floor (so nothing solid like concrete) and I can easily detect M3+ local earthquakes, M5+ from the rest of Europe, and everything above M6 globally (unless in the shadow zone). So, unless you need a very very low noise level for some specific project/purpose, I would feel that what you have should be enough.