The abs are composed of mainly fast-twitch (FT) muscle fibers. That means they're best suited for lower reps using heavier weight rather than tons of reps with just bodyweight. It's easy enough to add weight to "upper ab" exercises like crunches, but what "lower ab" movements? Try this:
In most gyms you'll see a medicine ball or dumbbell held between the feet for resistance during knee-ins, but since the weight is acting downward, this tends to over-activate the hip flexor muscles to keep the weight up off the ground. A much better option is to use horizontal loading using a cable, which provides resistance in the direction of the movement.