A 45-year-old man on ICU following a road traffic collision has a flail chest (right-sided, involving ribs 4-8) with an underlying pulmonary contusion. He is in pain despite IV morphine PCA. His respiratory rate is 28, SpO2 92% on 6 L/min, and tidal volumes are shallow. ABG: pH 7.33, PaO2 8.0 kPa, PaCO2 6.0 kPa on FiO2 0.35. What is the most effective analgesic strategy for flail chest?