Step 3: Finishing the box
Here is a close-up showing one half of the main panel. This well will hold one 18 cell sub-panel. Notice the little holes drilled in the edges of the well. This will be the bottom of the panel (it is upside down in the photo, sorry). These are vent holes to keep the air pressure inside the panel equalized with the outside, and to let moisture escape. These holes must be on the bottom of the panel or rain and dew will run inside. There must also be vent holes in the center divider between the two sub panels.
UPDATE: After using the panel for a while, I now recommend that the vent holes be increased to at least 1/4 inch in diameter. Also, to keep dust and critters out of the panel, stuff a little fiberglass insulation in the holes in the bottom rail of the panel. The insulation is not needed in the holes in the center divider.
Next I cut two pieces of masonite peg-board to fit inside the wells. These pieces of peg-board will be the substrates that each sub-panel will be built on. They were cut to be a loose fit in the wells. You don't have to use peg-board for this. I just happened to have some on hand. Just about any thin, rigid and non-conducting material should work.
To protect the solar cells from the weather, the panel will have a plexiglass front. In the third picture, two pieces of scrap plexiglass have been cut to fit the front of the panel. I didn't have one piece big enough to do the whole thing. Glass could also be used for this, but glass is fragile. Hail stones and flying debris that would shatter glass will just bounce off the plexi. Now you can start to see what the finished panel will look like.