I originally stumbled across the R'lyeh Calendar Stone from UndertakingFX back in May. Now that I actually have one I can safely say that the pictures don't do it justice. The plaque is approximately 1 foot in diameter and around 1.5 inches thick at its thickest point.
Put simply, it's a great prop with a huge amount of potential for customization and accessorizing. The calendar is cast from hydrostone and has a very nice heft to it, appropriate for something crafted from real stone. It displays nicely as is, but storing it packed in excelsior or raw cotton in a weathered shipping box would kick it up a notch. If you're really motivated there's no end to the ephemera you could produce, from excavation photos to dig notes. For that approach I would heartily recommend the work of Florian Mellies as an inspiration.
The calendar stone is available for $100 +shipping from email@example.com. I don't believe they have a website, but you can browse some of their other work in this gallery.