One simple solution is to glue a sheet of Dynamat on the cabinet wall, cut a hole big enough for the fan, and mount the fan on the dynamat. You can also glue Dynamat on any cabinet surface to damp its vibration, very effective. Take a look at their web site www.dynamat.com for more details.
BTW, Some Dynamat products are heavily discounted at Amazon.com.
BTW, Some Dynamat products are heavily discounted at Amazon.com.