We are adding a soundproof wall over an existing wall. We have an electrical outlet that we want to extend. This will be about four inches or more as we are also adding residual channeling then half inch drywall. How do we extend the outlet to be secure and safe?
Extension boxes stack
You can use multiple extension boxes in this situation -- I'd go back to the hardware store and get an extra extension box as well as a couple of mud rings of different depths if I were in your shoes. You may need to move the box to get enough wire length sticking out, though -- you need a minimum of 6" of slack with 3" of that sticking out the face of the box.