Handy Box Extension on a Plastic wall box
There are extension rings that fit over a standard single gang ring 



Like any metal box, you'd have to bond the box to the EGC (green / bare wire) with a grounding screw or clip.  

It will work just as shown in the photo for a single gang ring.  I learned in a comment below, you can rotate the box 90* and the four holes in the back of the box will line up for a double gang ring.