Arlington’s non-metallic BES series box extenders add height and volume to an existing electrical box for large items, such as a dimmer switch, in a shallow box.
Arlington's BES series box extender with an extra gang
The product adds 5/8-inch height and 5.3 to 10.5 cubic inches to existing box volume
BES2C couplers add an additional gang and 3.5 cu. inches in volume and uses standard or mid-size wall plates. They also are paintable and UL/CSA listed.
For multiple gangs, a BES2 and BES2C coupler(s) can be snapped together.