Here's a tip for getting your homebuilt box to look nice - Bondo. A while back, I ran into a cheap source of rugged boxes about the right size for tinkering, but they weren't exterior quality because there were gaps in the corners - pressed steel electrical wiring boxes, like you use in your house!

You'll need some Bondo (or some other brand of auto-body filler), a razor blade (the kind you use for scraping paint.), and fine sandpaper. The only thing that might be slightly difficult to find is the sandpaper, but I've been able to find stuff down to 8 micron at places like Lowes.

The razor blade is to give you a nice flat straight edge. Use it to level the Bondo while it's still gooey. Once it dries (I can't stress enough to let it dry thoroughly.), sand it smooth. Emery boards are great for roughing in the first application, but you'll want a very fine wet-dry sandpaper on the final application. (400 - 600 grit range or higher. Just don't start at 1200 grit.) After this, you're ready to prime and paint.

One other neat Bondo trick is filling accidental holes. Small holes will fill with no special work. If you're working with steel chassis, however, you can plug rather large holes. Go down to the hobby store and get a sheet of tin. Cut out a patch slightly larger than the hole, and solder it to the inside of the chassis using a very heavy duty soldering iron. (Guns don't work for this, you'll need a lot of mass in addition to a lot of power.) Make sure it is absolutely dry before you sand/apply the next layer, or it will shrink in a bit, giving you a slight depression that'll really show under a glossy paint.

Remember to work in a well ventilated area, and follow whatever directions are on the body filler you use. Also, don't rely on filler for structural integrity (this is obvious) or for RF shielding. (Sometimes a little less obvious when the box looks nice and smooth from the outside.)