First, put the number of batteries you want on some paper, and trace around them. Leave about 1/4" at the front and back of the batteries. This will be room for the positive and negative terminals.
Leave enough on the sides to wrap around the left and right batteries (this will be the sides of the holder. Note that I tried to get fancy and make front and back tabs from the same piece. I advise you not to try this. I messed it up and had to cut them off.
NOTE: When cutting this plastic, you need to score it pretty deeply in order to have it snap clean. This is somewhat dangerous. Make sure you cut it in such a way that, if the knife slips, it will cut away from you (and other people).
Cut the edges from a jewel case cover, so you have nice, flat stock to work with. Once it's scored, snap it off. If it didn't snap cleanly, trim very carefully with the knife. Put the template under the plastic to guide your cuts. Score and snap the piece you need.
Step 1: Forming the Battery Compartment
Place the piece of plastic over the top so that the same amount of plastic is hanging over each side. Use your torch to melt along one edge enough to bend the plastic. The plastic has a tendency to curl under the battery form a bit, which is handy for snap-fitting the batteries once it's done.
WARNING: my plastic caught fire once, so you should probably be prepared for that. Don't have any other flammables around and keep a fire extinguisher handy. But I was just able to blow it out)
Now you have a 2-sided battery holder.
Step 2: Forming the front and back
Next we'll put the metal components in place.
Step 3: Adding the terminals
To find the center of the terminals, dab a battery's positive terminal with paint (you might want a dead battery). Dry fit an end onto the holder and press a battery in place to transfer a dab of paint. Do this twice for each battery. Drill a hole big enough to feed a wire through each marking. I used a Dremel with a fine diamond point bit to do this.
The positive terminals will be our nuts. The negative are our springs.
Feed a wire through an end at the positive terminal. Wrap the wire around the nut a few times and solder to itself (in the picture, I first tried to solder the wire to the nut. It didn't work. Leave enough wire so you can attach to whatever project for one positive terminal. For the other, leave enough to connect to the negative terminal of the other battery. Use a small dab of hot glue to glue this in place.
Bend one end of each spring so it can be fed through the plastic. Bend a bit of the other end in to increase the area in contact with the battery. Feed the long end of each spring through the hole in the plastic. Secure the spring on the inside with a small dab of hot glue. Wrap your wires around the spring ends and solder.