ArtMetal

With some sheet metal equipment, this is a snap to make - layout the whole thing on a single sheet of copper, cut out the corners, fold on a brake and solder or rivet the four corners of the sides. If you don't have a brake, then it gets a bit more difficult, but not much. You can make the bends using two pieces of flat bar clamped together and a pair of sheet metal Vise Grips, or just carefully work the bends over the corner of a block of steel or even hardwood with a mallet.

Copper rivets are available already headed. Check out JayCee Sales online at http://www.rivetsinstock.com for the size you want.  For setting the rivets, you'll need a header and a bucking bar if you want them to come out perfectly formed.  For your project, you'd need to put the rivets through from the inside and then head them on the outside where you can reach them.  The bucking bar can be nothing more than a piece of heavy barstock with a depression drilled in it to index the head of the rivet, clamped in a vise so you can slip the box assembly over it.

Check out some of the metal working forums that the auto body and hot rod guys frequent for lots of good tips on working with sheet metal.  Try a Google search for "sheet metal fabrication" or some such to find tips and ideas.