If you just can't seem to hit it with press or squat variations (and you're crushing heavy extensions), you might try some heavy goblet squats. Concentrate on the desired area and screw with foot angle and width until you feel it.
Hack squats. They may seem awkward at first but you'll definitely feel it when you get the movement down. I've also had some success using a band around my legs above me knees when I squat
