I did this with my previous mailbox and really liked it, I no longer had to turn on the light in my truck to see if there was mail in the back of the box. Well the wife suggested that I get a larger mailbox, Now I am able to provide you with my version of the solar mailbox light.
Step 1: Tools
For this version of a solar mailbox, I used:
-$1 solar light from Dollar General
Step 2: Select Hole Saw Size
To ensure that your saw is the correct size:
-remove the post that the light came with (pull it off) discard this piece.
-remove the lens ( usually a 1/4 turn) SAVE
-place hole saw on the photo cell unit should be smaller ( mine is cutting it close, its a 2 1/8" but I have lots of this size and do not mine dulling the teeth).
-Place hole saw over lens ( should fit at least over the largest diameter of the lens not including the tabs). The closer to the size of the lens the better, it will give you more surface area to work with.
Step 3: Drill Hole
IMPORTANT: If the mailbox is not already installed, you need to determine which way the mailbox will be facing before proceeding.
Once you have determined which direction the mailbox will be facing or if it is already installed:
with the drill and hole saw: drill a hole in the side facing the equator. (if you do not know which direction that is wait till noon and drill a hole on the side that has the most sun light on it). I prefer to have mine closer to the back of the box so it is less likely to get hit by a package.
Step 4: Attach Light
-Reinstall the lens on the photo cell / light (if you have not already done so)
-apply silicone to back side solar cell trying not to get any on the lens, and around edge of hole created in previous step
-insert light assembly into hole and give a slight twist to ensure all areas are contacted. wipe off any excess. You may want to tape it in place with some masking tape, to ensure it does not move. I was able to turn the mailbox on it's side while it dried.
Once silicone is dry, you can install it, if it has not been done so already.
I tried to take a picture of what it looks like at night when you open the door, but my camera did not give me any good ones to choose from.
I like it, I no longer have to turn on the light in my truck to see if there is any mail in the back of the box when checking it at night.