Firestine

Paint the box with shiny silver spray paint. Apply the paint in very light coats because the chrome paints will have a tendency to puddle and dry with blemishes.

Belt Box 08 by winterjg, on Flickr
Find the center of the back plate and drill holes about 3/4" from the ends.
Punch holes in the ends of the leather strap.

Belt Box 09 by winterjg, on Flickr
Use the quick set rivets to attach the strap to the back plate. The strap should be loose so there is room for the belt to slide in.

Belt Box 10 by winterjg, on Flickr
You can see what happens when you are not careful with the paint job! But it still holds up to passing scrutiny. (I'll fix it anyway) I also recommend a clear coat so the silver does not rub off on your costume.

Belt Box 11 by winterjg, on Flickr
Screw the backplate to the belt box.

Belt Box 12 by winterjg, on Flickr
Sexy, isn't it?

Belt Box 13 by winterjg, on Flickr
Put your new box on your belt! (The other one is drying)

Belt Box 14 by winterjg, on Flickr
I hope this helped you a little and I appreciate any comments or suggestions that can make this better.

Cut the unneeded portion off carefully and clean up the edges. The posts for the screws will need to be lowered a bit more than walls to accommodate the metal back.

Belt Box 05 by winterjg, on Flickr
On the face there are 4 raised circles that will need to be sanded off.

Belt Box 06 by winterjg, on Flickr
Clean up the outside with a fine sandpaper and or #000 steel wool.

Belt Box 07 by winterjg, on Flickr

On to the Tutorial.
Items Needed:
Project Box from Radio Shack - 5" x 2.5" x 2"
2 thin leather straps - 4" long
Easy set leather rivets
Silver spray paint
Masking tape
Sandpaper / #000 Steel Wool
Ruler
Pencil
Dremel/Drill/Sander (or other cutting/sanding method)

Belt Box 01 by winterjg, on Flickr
The box comes with a plastic and a metal back and 4 screws. I prefer the metal back since that means there will be less silver spray painted plastic rubbing against my costume.

Belt Box 03 by winterjg, on Flickr
Mark the box 1" from the face with masking tape.

Belt Box 04 by winterjg, on Flickr

I wanted to come up with a method for people to make belt boxes that used basic skills and were not expensive.
The 501st CRL shows the following for the belt boxes:
"Dimensions are approximately 2 1/2" (63.5 mm) by 4 1/2" (114.3 mm) by 1" (25.4 mm) thick."

I was unable to find a usable item that would be readily accessible to as many people as possible with those exact dimensions. The best I could find was a project case from Radio Shack that is 5" x 2.5" x 2" that is easily modified.

Belt Box 02 by winterjg, on Flickr