I started with a basic plant window box that set the width and depth of my planter. I suggest using a planter box without any drain holes. If you get one with drain holes, simply plug them.
I created a small base that was the full width and depth of the planter using both 1x4 and 2x4 wood. I used the 2x4 as the vertical supports and the 1x4 as the base and to support the shiplap. (I tried to use all of the scrap wood that we had laying around the garage.)