I am putting up 3/8" plywood on top of old plywood in a carport ceiling. This will add to the thickness of the ceiling and bring the ceiling down 3/8". I would like to keep the old octagonal metal box which was nailed into a ceiling stud and which housed the ceiling light fixture. The old metal electrical box was perfectly flush before the addition of the new layer of plywood but will now be 3/8" below the ceiling surface. Can I just attach a 3/8" piece of plywood onto the stud and nail the box through the 3/8" piece of plywood into the stud, thus bringing the box flush with the ceiling? Or, is that not the right way to do this?

 

Just usea box extension such as this one.

 

That's the solution, thanks.