Place the stretched canvas face down on a flat surface so that the wooden support structure is visible. Measure the thickness of the wood used to construct the canvas frame.
Purchase a 1-foot by 6-inch (or larger) piece of wood that is of equal or lesser thickness to the canvas frame.
Cut the wood down the center to create two equal 6-by 6-inch squares.
Draw a straight line along each wooden square from one corner to the opposite corner. Cut along the lines to create four triangles, each with one 90-degree angle and two 45-degree angles.
Fit the 90-degree corner of each support into the 90-degree corners created by the frame. Secure the supports to each corner with brad nails (panel pin nails).