-empty bullet shell (300 Win Mag)
-pen / pencill (some fancy expensive gift ones to be marked for life)
-a collection of tamping tools lying where I've smoked last (and by pure chance I found it while looking for a tamping tool, only to lose it again)
-someone else's finger with the necessary repercussions
-and my latest - I turned one, similar to my trusty 300 WinMag empty shell from a piece of wood that was too small for any other purpose.
I use my finger all the time, after you tamp too hard a few times you learn your lesson. Other than that I've used a nail, a twig I found while on a walk with my wife and son, and a golf tee. I have like 15 Czech tools so I usually don't need to use a substitute, but sometimes I find my finger works best, especially when you have an uneven burn going on and you want to coax the edges toward the middle to take a light better.