To start I measured the height and width of each side of the electrical box. I added 3" to each side to account for joining the sections.
The box is actually a rectangle - 31" on one side and 33" on the other. To build the box I made 2 identical panels at 34" wide and 2 at 36" wide.
Since this is a working electrical box and needs to be accessible to the power company, I needed to build something that could be easily removed. I decided to create 4 panels and join them with removable pins in the corners.