A box to fit over the entrance with a queen excluder as entrance/exit. Not rocket science here. Just need some form of secure attachment.
They work but are a pain as they need attention for closing/opening morning and afternoon to allow drones to fly. Not so much use, I feel, as simply checking for swarm cells and artificial swarming, before a trap is needed, is the way to go.
I reckon they are generally only of use in emergencies and for those beeks who try to subdue the swarming instinct by destroying queen cells at inspections - which just doesn't work!