Step 6: Glue/Nail on Support Rail
My support rail is long and thin (scrap wood 1/2" thick) cut into two 4' feet strips and pre-drilled (with a small drill bit) in four places ready for screws. I apply glue to one side of the support rail and screw onto the inside of the board with my 1/2" inch gap in mind. Pre-drill your rail and use 1 1/2" inches screws.
Note: If you select a wood strip support system instead of plywood, make your support rail the entire length of the side boards. You can't afford gaps with a wood strip support system.