If you do enough remodeling work, you’ll eventually encounter a situation that calls for extending the depth of an existing electrical box to match the thickness of a new wall. There are commercial extensions available, but sometimes, the fit needs to be a little more exact than that afforded by the store-bought variety, and the outlet or switch ears really need to mount tightly to the box. In this case, I make a custom thickness extension from a plastic box.
As shown in the drawing, I nail a box to a 2x block and clamp it in a vise. Then I draw a cutline around the box to mark the required thickness, and I use a fine-toothed metal-cutting blade in a reciprocating saw to slice off the extension.
Because they are standardized, the screw-mounting holes align perfectly with the mounted box. I use mounting screws that extend at least 1/2 in. into the existing box.
Jay Lane, Portland, OR