Let’s assume you want to add an extra box when you’re doing your electrical. The other boxes were installed in the right places during wall construction, but somehow you forgot one. (This would never happen to professionals, of course. [cough])
Don’t worry, there’s an easy fix. What I do… [cough] or would do if this ever happened to me, is use ¼” steel rod to pin the box to the earthbags. Cut two 7” long pieces of steel rod. Pound the rod in at opposing angles so the box won’t pull loose. (See drawing.) For example, angle one pin up and one pin down. Bend over the last 1-1/4” so it’s flat against the back of the box. This creates a very secure box. Just remember to have the front edge of the box protrude about 1-1/4” beyond the earthbags to allow for plaster thickness. Now you’re ready to run your electrical wire.