Measure the back middle of the play prop, making sure to stretch the tape measure from the top middle of the back of the prop to the bottom middle of the prop.
Take this measurement to your local home construction store and ask a store representative to cut strips of crown molding -- or wood strips that are similar to this -- in the length of your measurements.
Lay the prop on the ground so that the back faces up. Nail the strip of wood to the back of the prop at the top and bottom frames. This strip of wood will act as a vertical brace.