In short why is Silicone better especially thermal one then Sorbothane.
For one Silicone propagation frequency is considerably lower 9.5 Hz vs 19 Hz Sorbothane and 19.8 rubber. Magnitude is lower then rubber but bigger than Sorbothane but on the other hand harmonic propagation is both higher and uper compared to Silicone. This changing with introduction of slim 1mm metal casing around Silicone and now it's best by all metrics. They all lose properties and propagation moves higher with pressure applied so a tin metal casing reinforcement is more than advisable in any case, again Silicone is best. Energy doesn't disappear or can be dispersed entirely so cheneling it to under hearing threshold is the most important property in sound related use cases which is not the case with either rubber or Sorbothane and with Silicone it remains to be even with large amount of mounted pressure.
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Long story short make your own acoustic isolators as described not only they will be cheap and nice looking but also most efficient. Don't expect industry to do it for you, there is no money for them in doing that.