Preventing Wandering Off

Instance maps should be created with preventing players wandering off in mind, but until adventure mode exists only bedrock is really going to stop a determined person.

Define an area with its opposite corners that encloses where the player is allowed to go? Warn them if they're nearing the edge, and count down once they're outside the boundary. If time runs out, teleport the player back to the entrance or out of the instance entirely?