It's a plastic box with little ears that flop out and sandwich the box to the wall, or the cabinet back - same principle as a mobile home receptacle, easy to use, very common in kitchen remodeling and passes inspection and codes. Just do it like it should be done. Covering any electrical connection, including an outlet, prevents necessary heat dissipation and blocks access for future inspection or maintenance. Correcting this issue requires understanding. What is the most efficient way to rough in wiring and later install receptacles in the back of cabinetry for built-in appliances? In the past I have just left whips and then come back and cut in the receptacle. But it's a pain because you're hunched into the cabinet opening doing the work. I've done this before but this time there are cabinets involved.
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