Here is a training bench that lets you work your core the right way. The Vicore Fitness Core Ab Bench is ideal for proper sit ups and crunches. It features 75mm diameter tubing with 11 gauge steel. Its rubber foot pads are safe for all floors. You also get a handle to carry the bench around conveniently.
The above GIF gives you a better idea how you can put the ab bench to good use. More info is available here.