Cut an acrylic sheet to the size of the solar panel, leaving about a half inch lip on the bottom to attach a photocell for measuring brightness outside.  The photocell will allow you to keep the Neopixels off during the daytime to save on battery.  Use an exacto knife and a piece of scrap wood for a straight edge to cut into the acrylic sheet.  Once cut, place the acrylic sheet in between two pieces of wood along the shear and apply downward pressure to break off the measured section.

Measure and drill two center holes for attaching to the mailbox post, four corner holes for attaching the solar panel, one offset side hole for the wire coming out the back of the solar panel, and one center bottom hole for holding the photocell.