Building a metal mailbox is pretty complicated compared to other types. With the amount of effort you have to put forth, I recommend you try a wooden mail box unless you’re familiar with metal work. They look nicer, are easier to build, and require fewer materials. However, if you’re dead set on a sheet metal mailbox, just follow these simple instructions.
Take your sheet metal and cut a 23” x 23” piece out of it. Use a file to take the sharp edges and burs off. This is going to be the top arch and sides of your mailbox.
Next we’re going to make the bottom. The dimensions should measure 9” by 21”. Next we’re going to bend the top piece so that it forms the arch you’re familiar with. Use a perfectly round object to create an even curve. You can use a large pipe or something similar. It’s also possible to form it by hand, but it’s much harder to create a smooth curve by hand.
Now we’re going to attach the top to the bottom. Make a ¾” bend on the bottom two sides of your top piece at a 90 degree angle. Take your bottom piece and place it inside the arch so it’s resting on the sides that you just bent. Spot weld the bottom piece to the top using as many welds as you can. You may not be able to reach the spot welder all the way to the center of the mailbox depending on the type of machine you have.
Now it’s time to make the backside of the box. Trace the backside of the mailbox onto a piece of paper with a pencil. Add ¾” extra all the way around so you can bend it like you did on the sides. Spot weld the bended ends to the backside similar to the top and bottom.
The front side of the box is almost the same as the back. However, with the front you of course need a hinge so you open and close it.
Lastly, trace a flag style you want. It can be a traditional flag or you can get creative with it. Sometimes it helps to have a mailbox theme picked out so it can match the flag. For instance, if your mailbox is designed like a lighthouse, the flag can be a lit light bulb.
In order to prevent the metal from rusting, you’ll need to paint it. Spray paint the inside because those areas are harder to get to. The outside can either be sprayed or hand painted to create an artistic theme.