Assuming I can find the duplex inserts...

Standard Leviton (and other manufacturers) item. And they can be had with 6 ports, not limited to 4 as I said above.

I've had good luck with a 4-square box with a 1G reducing mudring on it...

You might not know but they do make covers for a 4-square (otherwise known as a 1900) box for surface mounting. They are called austin covers. Use that instead of a mud ring and wall plate. Then too 1900 boxes are available in different depths- I like to use a deep box. And there are extension rings that are basically a 1900 box without a back that you can stack to make the box deeper. Also, instead of a 1900 look at a 4-11/16 box.

-Hal

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