I bought two pieces of marine grade plywood so we could leave the boards outside in the elements.
I then applied two coats of blackboard paint to both sides and edges of the wood. This took days because it is an oil based paint and needs to dry thoroughly between coats.
I decided to leave two sides completely plain and to paint 'drawing prompts' on the other two sides to encourage creativity. I painted some feet along the bottom edge of one blackboard and an incomplete landscape scene along the edge of the other - all with exterior paint. The idea of painting on some 'drawing prompts' is to encourage the children to finish the pictures.
Have a go at making your own blackboard. Your kids will love it !!