Metal electrical box extenders
The National Electrical Code does not allow devices to sit back inside combustible material.
The pertinent article attention to the second paragraph:

314.20 Flush-Mounted Installations.  Installations within or behind a surface of concrete, tile, gypsum, plaster, or other noncombustible material, including boxes employing a flush-type cover or faceplate, shall be made so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of the finished surface more than 6 mm (1∕4 in.).
Installations within a surface of wood or other combustible surface material, boxes, plaster rings, extension rings, or listed extenders shall extend to the finished surface or project therefrom.

They make cheap box extensions that are basically a piece of stamped metal with a slit in one side that can be installed without de-terminating the device. Just remove the receptacle, slip the conductors through the slit in the side and reinstall it flush with the surface.
Good luck!