After MUCH trial-and-error (a lot more than I had anticipated) I think I have finally come up with a good method for making coasters. They are durable- I've tested them with hot coffee mugs and cold glasses of ice water. Not dishwasher safe but can easily be cleaned with a damp cloth. They are backed with cork to prevent sliding. I even put a few sets up in my Etsy shop.
I have a few more designs in the works and plan to do a few non-Fenway Park sets as well. Come back soon!