as peter said, any rigid material would do…
however, i think its maybe an idea to create cheap as chips one or a nice office one.
cheaper one could be on correx scrap, folded like a counter top sign. sits on a pub counter with "today’s specials" at the top and they write on the soup of today etc on the dry wipe part.
office one could be a coloured bit of dibond, raised on four ally locator’s, or just plastic ones with title and area below to for the dry wipe panel… that kinda thing….
great thing about left overs/scraps as you can be as creative as you like, as long as you make some sort of profit. 😀