MEASURE AND CUT
Measure from the top of the frame to the top of the 1' x 3' panels. Cut (4) 1" x 2" boards to this length. I actually cut mine 1/4" shorter and at a 45 degree angle to get rid of the corner sticking into the box.
ATTACH TO THE PANELS
Line up the corner so that the panels are nice and flush. Clamp down the bracket as shown. Drive in the first nails as close as you can to the clamp because they are going to try and separate the the board from the panel. After the first panel is nailed to the bracket switch the clamp to the other panel in that corner and nail that panel to the board. There is no need to nail these corner brackets to the frame. Repeat these steps for the other three corners.