A patient is admitted following a road traffic accident. He has sustained significant blunt injury to his head, chest and abdomen and has a Glasgow Coma Scale score of 8/15. His saturations are poor at 89% on 15 L of oxygen a rebreathing mask. You note bruising around both eyes and blood-stained fluid issuing from his left ear, which forms concentric circles when dripped on a white sheet. You wish to suppo his airway to improve oxygenation. The first choice of airway adjunct would be