Here is a simple shadow box that you can make from an Altoids Smalls tin. You could make it from one of the larger Altoids tins if you wanted to, as well. The steps would basically be the same.
Step 1: Get the Tin and Remove the Paint
Find a tin and some tools to remove the paint finish. There are a number of ways to remove the paint. You could just have at it with some sand paper or steel wool (wear gloves). This is time consuming and messy. You could soak it in some paint striper or acetone, but you will still need to scrub and sand some of the paint off. Acetone just softens the paint and doen't make it just come off. A good method is using a Dremel with the buffing/polishing pads. These pads are simular to Scotch-Brite scrubbing pads (which would also work by themselves). There are a number of other attachments for the Dremel which would remove the paint, but I like the pads.
In the end I didn't do any of this. I have access to a small sandblaster and it made quick work of it. It left the box with a nice matte finish. I realize that few people with have this kind of tool around, so you'll just have to do your best.
It is also possible to just paint over the whole thing. I would still sand it mostly to help with paint adheasion. Painting it would probably be best done later after the next few steps.
Step 2: Cut a Window in the Lid
Next, use masking tape to "window" the opening you are about to cut in the lid. The tape will serve as a visual guide for the Dremel cut-off wheel. Try to make it as balanced looking as you can. Then, using the Dremel, cut the opening. Don't try to cut too fast and make sure you are wearing protective glasses and gloves. The Dremel can throw a broken cut-off wheel faster than you can blink and the box will have sharp edges from the cut. It is, after all, made of sheet metal.
File the edges with a fine needle file or diamond hone. Remove the burrs as best you can without bleeding too much.
Step 3: Surface Finish
Remove the tape. Now it is time to decorate, paint, electroplate, or otherwise on the box. I happen to have a copper electroplating kit and after about 10 minutes in the solution I had enough of a copper color to satisfy me. It is not very thick and will wear eventually, but that is just part of the charm. There are really too many options here to list them.
Step 4: Glue in the Window and Line the Box
Now all you need to do is cut a piece of acetate, CD case, Lexan, or Plexi-glass to fit inside the lid. Acetate would probably be the easiest and you can also find a piece of plastic from discarded packaging. CD cases shatter very easily and can crack. Lexan and Plexi can be cut with a hand saw or draw knife. I used a draw knife to cut a piece of Plexi. When using a draw knife, I strongly recomend using a ruler or other straigh edge. Scribe it a few times and snap it clean.
Glue the window in with a hot glue gun or RTV. I prefer RTV, but it takes a while to dry and it can get all over everything. If you get glue on the window while installing it, don't try to clean it up. Wait for it to dry and polish it clean and clear with a plastic polish compound. Or put a new one in and be more careful.
Finally, line the box with felt or a nice handmade paper. You can also stick an adheasive magnet for business cards to the back of the box.
That's all! Whatever you put in the box will need some kind of mounting. Poster Putty works well and magnets stick to the box great.
My blog: Ossa Piscium