I'm trying to remember the name of the extension collar that is fitted between a surface mounted accessory and a flush box to increase the depth behind the accessory. Basically I have a dimmer switch that is too deep for the 1 gang switch box. I've used these extensions many years ago but for the life of me cannot remember what they are called. Looking to buy one from a wholesaler tomorrow and don't want to look a complete prat. Any help much appreciated.