In addition to my printer in the enclosure, I also wanted to isolate vibration through the enclosure as well. So I modified the feet for the printer box to accept a 50mm x 50mm square of sorbothane pad.
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Sorbothane Pad (.375in, 4in x 4in, 50 durometer).
My rough calculations said that 50 durometer would be adequate for this application, but you do you.