Add the Threading Rods
Once the angle irons are attached, run two threading rods down each side of the plywood. The threading rods will be bent and used to hook over the crossbar. Before inserting the rods through the angle irons, bend them into shape.
Screw one washer and nut to the middle of the threading rod, then place the bottom of the threading rod through the holes of the angle iron. Once the threading rod is below the bottom angle iron, screw another washer and nut onto the rod. You now need to spin the threading rod to get it to go all the way through the angle irons. You can do this by hand or use an old carpenter's trick. Place the bottom end of the rod into an empty drill head, spin the drill backward slowly and the rod will come through the angle irons. Once it's fully through, tighten the washers and nuts to secure the threading rod to the angle irons.
A good way to do this is by using an old hose. Take a utility knife and cut the metal end off the hose, then slide the rod into it. Bend the rod while inside the hose -- this will make it easier to grip and protect your hands. After the rod is bent, use a utility knife to slice the hose open around the hook. Cut off the hose and use a second piece to do the same for the second threading rod.