I used a bobbie pin I found under the carpet in the trunk of my '67 Mustang. I bought some cotter pins but left the bobbie pin in to see how long it would last.
Answer: Three weeks.
I had to use it to jam in the place where the bolt that held the linkage to the top of the transmission used to be.
The bolt fell out and was gone, and the pen was just the right size to bottom out in the hole and jam into place on the side of the hump that ran down the middle of the car.
Perfect fit!