I was out doing some pinch work today and wanted a pony clamp to work on my thumb strength, but I didn't have one around. Then I started looking around the garage for something to simulate the movement of using a pony clamp. I found a pair of jumper cables and it worked out great! Here is what you do....
Take your cables and wrap a few rubber bands around the head of the clamp, make them tight around it. As you use them you can add more rubber bands to make it harder to close or take some off if it is to hard for you to close and work your way up.
Once you get the bands on the clamp position it in your hand with your thumb on one end and two fingers or more on the other end. I prefer one or two fingers as you can cheat this with more fingers which will make the weight on the thumb diminish.
Then all you need to do is simply close the cable pressing your thumb to your fingers, hold it closed for a second or two and continue. I like to go for 3-4 sets doing 4-8 reps to get the thumb nice and strong. Now you can go train your grip with out having to go buy some other piece of equipment that you may never need or use!