I have a room with plaster walls that were re-coated with plaster. The boxes are, predictably, recessed. Although they probably meet requirements, I'd like to bring the switch and outlets flush. I have a mixture of old, stud-mounted metal boxes and add-on plastic ones.

I tried a couple of washers behind the yoke on one plastic box, but it did not seem to help. Today I poked around a bit and found this:

http://www.aifittings.com/products/spec-sheets/BE1.pdf

Is this the best way? Are there alternatives? I don't understand how this will work with the differently-dimensioned boxes.

Thanks