One of the quickest and easiest moves out of the under hook is the traditional high crotch. It's used best as a re-shot, after your opponent takes a bad shot or a shot that you have down-blocked.
While you have an under hook, make him circle toward you with your free hand hooked on the front of his head. Usually this circling is to make a foot step and be closer to shoot on, but this time it's to make him square up his stance so the shot is a little less awkward. Shots are also easier when your opponent is busy trying to stabilize himself.
You'll lower your elevation and throw the under hook over your shoulder as you penetrate to your high crotch. This should be one quick motion.
Always remember to finish your shots while drilling. If you don't finish in practice, you'll hesitate in matches, and that's when your opponent will start to defend. So practice converting to the double, driving sideways, and finishing the takedown as fast as you can.