This weekend I chucked this together using things I had kicking around. I'm not in the least bit artistic so the lettering is done with laser printable transfer sheet. I love this stuff.
I found a small piece of slate, cut it down with an angle grinder, notched it for screws and also used hot glue to stop it moving about.
Having a lump of stone that you write on with chalk seemed the appropriate thing for a fantasy LARP, rather than an LED matrix. :-)