Have you tried Vertical Lying Leg Thrusts yet? These are a great target exercise for your lower abs.

Begin by lying on your back; I like to do the movement on a flat bench, but it also works on the floor.

Now lift your legs until they’re vertical. Tuck your hands under your buttocks, and make sure your lower back is flat on the ground.

To perform the movement concentrate on tightening up and squeezing together the muscles in your lower abs.  Your legs and your lower torso should move upward toward the ceiling.

Remember, the feeling you’re after is one of tightening and squeezing in your abs; and it’s this squeezing that should lift your buttocks off your hands.
 
Lower your torso back down to complete one rep.  The most important thing in lower ab training is not how many reps you do but how hard you squeeze.