Drill two holes in the side of the enclosure box to run one wire from the solar panel and another wire from the mailbox, which will contain internal wires for power, ground, the NeoPixel strip, and the door switch.
Use a drill press vise to hold the box in place while you drill.
Drill two holes through the back of the box for mounting to the mailbox post with wood screws.
Next, drill a row of small holes at 1/16" diameter across the bottom of the enclosure lid. This will help keep moisture out and allow for water to drain if the box happens to leak.