If you buy a clip-on light for a dartboard, they usually attach to the rim of the board. This is a problem if your board is inside a cabinet.

This block will screw onto the top external edge of your cabinet so you can attach your light externally.

The brick is 40mm front to back with small chamfered recesses, to hold a metal sprung clip of the light bracket. It'll take brackets up to 24mm wide. The 9mm diameter turret on top holds a 4x35mm countersunk wood screw high enough so the tip of the screw will penetrate about 9mm into the cabinet. It also assumes there's a hole in the centre of your light bracket that this fits into.

I've uploaded photos of the light bracket we've bought for an idea of what you need.