One upturned cone point in the center of the back edge. A 1/2" to 1" cone should do the trick.
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If you want the entire speaker to be isolated from sending unwanted vibration into the shelf and other connected shelves and components, I would recommend two large size compliant isolation feet under the rear of the speaker and two smaller size compliant isolation feet under the front of the speaker.
Disclaimer: I am a manufacturer of vibration control products