Yes, I've made one.
I used water resistant MDF. Fairly thick as it was going to be used outside and needed to be sturdy.
A frame made from planed timber (probably 18x44mm) screwed to the back of each piece and extending down to make legs. - painted the board first all over with 2 coats of chalkboard paint, then 2 small brass hinges into the tops of the frame.
Drilled a hole in each leg below the level of the boards and a piece of cord across each side to restrain them at the right angle when open.
I still have it buried out in the shed. Will look a bit tired now as it hasn't been used for ages, but I could photograph it later if you are interested.