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https://mctroniktek.com/amazon-devices/voice-dock-flush-mount-in-wall-bracket-for-amazon-echo-dot-2nd-generation
The Vanguard Dynamics Voice Dock mounts an Amazon Echo Dot securely onto any wall or ceiling. Installed in every room, it creates the ultimate custom integration voice control smart home by harnessing the technology of Amazon Alexa. Use the Amazon Echo Dot as an in wall volume control as well.

Cut Out Dimensions: 3 3/4″ (96mm)
Trim Cover Dimensions: 4 1/2″ (114mm)