Try the new amazing "Mel's Gym Ab Roller"!
Warning: I could only do TWO of these before I fell on my face and my abs are sore.
Take a full length or short olympic bar and put 10 pound plates on each end. Don't put clips on. Find a clear area on the gym floor. Stand in front of the bar, and grasp it as if you were going to do a straight legged deadlift. Roll the bar out in front of you until you are in a pushup position. If you haven't fallen down yet, using your abs (heck, use anything that works), pull the bar back towards your feet until you are upright again. Bend from the waist during this exercise and keep your knees straight!
If you can't do this, try it on your knees, or try to reverse the motion before you are in full extension. It's still quite a challenge!
Mel