Starting with a second stack of cut wood, fit, drill, stain and assemble the upper and lower sides to the metal frame. The lower sides are cut the 1/2 inch extra depth to accommodate the added width of the 1/4 angle iron.

This a simple step. Straight forward cuts and simple bolt and screw assembly. Take note. Everything else hangs from or bolts to the sides. It's important to get everything level and straight. An error here is something you will have to work around or compensate for on the rest of the project.