This is actually my second disc, so I have come up with two different ways to hold the batteries - I'm sure there are others.
Because of the reduced amount of space we have within the disc, traditional battery holders just won't fit. On my first disc - I happened to have a broken toy that took AAA batteries. I removed the metal tabs from the battery compartment and used them for contacts.
When I started working on my second disc - I wasn't so lucky. Also - because I'm writing an Instructable, I wanted something that would work for everyone. I'll describe the second method, the method using the metal tabs is extremely similar.
Please note - any suitable battery source from 4v to 5.5v will work for the provided chips. AA batteries proved too large, rechargeable batteries would require 4 to meet the 4v minimum, and small LiPo batteries were too expensive and required special chargers. For more detail on what you could do with LiPo batteries - see the NetDuino
For this Instructable - we'll be using half of a pen spring for the negative end of the battery and one of the metal clips from the old battery compartment for the positive end.
Because of necessary placement of certain items - our batteries have to fall into specific locations. The momentary switch is located at the top of the Disc. The battery compartment is located at the bottom - this is where we'll be placing our MCU. It is easiest to place our LED Driver as close to the MCU as possible. Because of this - our batteries will be broken up a bit. See Picture 1 for placement of the batteries.
The two batteries on the left half will be chained together with a larger connection looping around the top half of the disc to the battery on the right. Leave the leads from the bottom left and upper right batteries long enough to reach the MCU/Max7221 area at the bottom of the disc - leave yourself some extra wire to avoid having to add extra later on.
PREPPING THE SPRINGS
Start by cutting two springs in half. Next, solder a wire to the cut end of the spring. Once this has cooled a bit - coat 1/3 of the spring (starting at the solder joint) with a bead of hot glue. This will help the spring stand above the disc a bit when we put it in place.
PREPPING THE CLIPS
When we gutted the disc in step 3, we saved the metal clips from the battery compartment. Pick three clips that don't have any solder, or are as clean as possible. Start by bending the small tab up - this is the surface the "+" end of the battery will touch. We will be using the large for the wire connection. I found it helpful to tin my wire, then place a blob of solder on the large tab and then solder together.
ATTACH YOUR WIRES
Figure out how much wire you'll need between holders and solder them together - this will make the next steps easier.
MAKING THE HOLDERS
Now that you've identified where the battery holders will go and your clips and springs are ready - let's put them in. You'll need your hot glue gun.
I like to start with the clip rather than the spring. Place one of your AAA batteries in the provided area. Hold the clip (with wire lead attached of course) and place against the "+" end of the battery. Once you've decided where the clip will go, move the battery and apply hot glue to the BACK of the clip. Any glue that ends up at the front of the clip will need to be removed for a good connection. Allow the tab to cool a bit before installing the spring. To avoid any accidental shorts, place some hot glue on the edge of the tab facing the outside of the disc.
Now that the tab has cooled, put the battery back. Compress the spring against the other end of the battery to get a good idea of placement. You want the spring to be tight against the battery (although not too tight). Glue the spring in place - making sure to not get any additional glue on the spring as this will prevent it from compressing.
Once the spring has cooled, verify that the battery is held securely. I've found that it can be helpful to place additional hot glue around the outer edge of the battery to prevent it from slipping out. Make sure you don't put too much in this area because you'll be placing your outer LEDs next to the clips. You can always come back and shave off the extra later if needed.
Repeat for the remaining two holders.
NOTE - don't leave batteries in the holders until they've cooled for a while - it may push the connectors loose.