You will need: a sheet of marine ply, undercoat, and Rust-Oleum Chalk Board paint.
I got the hardware store to cut the marine ply sheet for me. Top tip: measure your car before getting the board cut to make sure it will fit.
Once you have your marine ply to size it is as simple as prime, paint and hang outdoors. We attached ours to two metal garden posts which were hammered into the ground.
The Chalk Board paint goes on smoothly. After a few coats let the paint dry for a week and then season the board by covering entirely in chalk and leaving for a few days. Ta daa!! You are ready to start drawing.
I put our board in a spot that gets afternoon sun in winter but is lovely and shaded in summer, now the chalk dust and the children are all outside. Perfect.
Rust-Oleum supplied the product for this road test but all opinions, as usual, are entirely my own.