What is the correct type backing box to put on a wall for wall lights.

The type of light I will want to fit would be probably one of the Ring
make lights (as sold in Homebase, B&Q etc) which are typically mounted
12 to 24 inches below the ceiling.

I lokked into one box & the instructions showed a backing box in the
wall without stating what type of box it should be.

The mounts on these lights appear to have holes 2in apart or so, so
bringing the cable out through a standard square backing box gives
mounting problems for the light fitting.

TIA for all help