Measure and cut a piece of MDF 18 by 13 inches, using a circular saw.
Draw two right triangles on the MDF. The triangles should have a 12 inch base, a 13 inch hypotenuse and a height of 5 inches.
Glue the long side of each right triangle. Place the rectangular board on top, with a triangle at either end. Fasten the boards together with two wood screws in each triangle.
Measure and cut a piece of MDF 16 1/2 inches long and 2 inches wide. Glue the ends of this piece. Place the MDF between the triangles to form a brace between them on the back of the foot rest. Screw the brace in place using the long screws.