That might be hard to do because Cuneiform tablets were also the first, and generally the best, form of encryption during the ancient era's. Basically, you could write them down on these tablets then, if I remember correctly, seal them between two tablets. That way, if the message came to you and it was opened, which you would have to do with a chisel, which would be very obvious to the person receiving the message, you could quickly realize that someone knows what you're planning and act against it. Cuneiform might make sense for the game, but at the same time it might not. Cuneiform was used in between the 31st Century BC to the 1st Century AD. But even by the 1st Century, and for a while before that, people were using papyrus to write things down, rather than the tablets (though that is mostly speculation, don't quote me on that, not an expert in that field). Coming across one randomly would be extremely rare, and most who would actually have a cuneiform tablet would realize that it's some form of writing, but perhaps not what it says. So, if you wanted to go down that route, perhaps a rich collector or merchant in town has one and you have to haggle for it?