Some time ago I made a device. Imagine a chair with arms. Take the seat out, and bolt the back and arms to a wall. Position it so when you are 'sitting' in the chair (your forearms on the arms of the chair) your legs are off the ground. Now lift your thighs until your knees are about up to your navel height. Repeat. Great for working the lower stomach muscles and if you twist back and forth, good for the side muscles of your stomach as well.
I also put upright hand grips on the ends of the chair arms.
These leg lifts are commerically available, but I made my own.
Sorry no pix as I tore it down some time ago. Got in the way in my shop during a money making project and never put it back up.