You can buy big sheets of template plastic at Joann's and Walmart (if your Walmart still has a fabric department). The cost is not prohibitive and they cut easily with scissors or an exacto knife. Just remember not to use the GOOD scissors!
If your knife is not working on the ice cream lids, there is an attachment for those little clover irons that is a heated knife that kinda melts through the plastic. I'm sure you could find something along those lines - maybe convert an old wood-burning tool to the purpose or something.
Good luck!