Paint the plywood sheet with blackboard paint, leaving a blank margin of 1.5cm on the edge. Follow the manufacturer’s instructions for the second layer. 

To prepare the frame: cut out the base of the crates. With a Stanley knife, cut out 4  x  3cm wide strips (1.2 inches) : 2 of which will be 24cm (9.6 inches) long and will form the 2 widths and 2 of which will be 29 cm (11.6 inches) long and will form the lengths of the frame. 

To make the corners, cut the edges at a 45° angle. Pre-assemble the frame to check that each strip joins in at the angles; they should extend beyond the painted plywood sheet by 5 mm on each side. 

When you are happy with the result, apply wood glue on each strip and glue to the painted plywood sheet. Leave to dry overnight.