The recent rash of great battery-holder Instructables has inspired me to share my own method. This requires a few specialized tools and dexterity, but I'm sure there are plenty of Instructabler's out there with the right stuff.
*There's a bunch of words in this Instructable, but you can probably just follow along with the pictures. :)
Step 1: Tools and materials
double sided copper clad board
3mm PVC foam board
bit of wire
rotary engraving tool
Step 2: Sizing things up
The first thing to do is cut a strip of copper clad. For best results it should be no wider than the thickness of the cell, and possibly a bit thinner. It depends on how stiff your copper clad is, because the board, itself, will be creating some spring action. I'm using 0.06" FR-4, which is pretty sturdy stuff. So I cut a strip of board a bit thinner than the battery.
You need to cut it long enough to make three parts. You'll see them later. Just cut a nice long strip about twice as long as the battery to be safe.
Step 3: Now prep the board
Step 4: Cut the the board into 3 parts
You cut off each end of the board about the length of the diameter of the battery, maybe a tad longer. In this case, the battery is 18mm in diameter (an 18-650 cell is 18mm wide and 65.0mm long)
Then from the middle section of pcb, you need a length that is at least a half centimeter longer than the cell. In my case, I made it extra long cuz that's where I like to put a bit of header as a plug.
Step 5: Preparing the pieces before soldering
Step 6: Optional
You can just solder wires, if you want.
Step 7: Time to solder!
The trick to soldering these straight is to start with a small blob at one edge. Eyeball and readjust until it's right. You want the end piece to be almost 90 degrees, but angled ever so slightly inward.
Once it's in position, solder a blob on the other edge. Then go to the the inside and solder the entire seam, leaving a nice bead of solder. The inner edge has to bear the stress from the battery, so you need to leave a pretty decent bead on that edge. Finally, go back and finish soldering the outer edge all the way. If you do it in stages like this, you don't risk reflowing the entire thing and moving the board.
Step 8: Anode
Then solder the end piece on just a bit inside the line. You want it to be slightly too short!
As with the other end, angle it a bit inward.
Be careful to keep your solder bead from splotching across the line you engraved, earlier.
Step 9: Pretty close to done
I soldered a small jumper wire to connect the anode tab to the positive pin on my header port. The rest of the surfaces on this holder are all in continuity with the cathode... so this wouldn't be a good battery holder for a multiple cell battery in series without some minor modification!
Step 10: Flex
This is why you need to solder the end pieces angled inwards just slightly AND closer together than the battery will actually fit without flexing.
When the battery is stuffed into the holder, the long section of board will flex slightly, keeping positive pressure on the battery terminals.
Step 11: Making the spring clips
To do this, you cut a piece of foam board and heat it up with a heat gun. Then wrap it around the battery.
After you get the shape right, you can cut the tube down and cut it into rings. I had some rings left over from the last time I did this, so I used those.
Put a ring on each end of the battery. Put a dab of hot glue on the bottom. Then stick the battery in the holder!