This is a fantastic idea! I'm not really a "World" coin collector but I have a suggestion... You can find ancient Greek coins that feature Homer on the reverse. No they're not very large but several examples are currently for sale on Vcoins, one on FORVM Ancient Coins...likely more out there. I just checked those two websites really quick. They're from the second century BC so perhaps in order to keep it in the time period an artifact would be best. Very cool Vonnegut is on the list! My favorite author.