Summary

This is a 3D-printable arc reactor sized to fit a neopixel ring.

Instructions

Printing this should be fairly straightforward. As mentioned in the description, the only thing that needs supports is the battery cover. The rest should have no surprises. Use whatever fill you want, whatever material you want (just make sure to use white or clear for the lens), however many shells/perimeters you want, etc. I experimented with printing a solid clear plastic lens and this produced the nicest result, but it was super hard to control warping. If you can conquer warping, then print it solid. You'll like the result.
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The basic print consists of one battery cover, one lens, one front, and one of either of the backs (with or without holes, depending on your plans).
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Bill of Materials:

some really small screws with matching nuts. Maybe locking washers if you want. I have a kit of tiny screws that I used and the ones I used in this project claim to be of size 2-56 x 1/4", whatever that means
one Gemma
one neopixel ring (size 16)
one individual neopixel mounted on a round board (the Flora ones)
one 1000uF electrolytic capacitor
one 300-500 ohm resistor (anywhere in that range)
150 mAh lipo battery with charger (Adafruit). Project is sized for this battery.
optionally, a switch and a button. I do not have part numbers. I sized this for stuff I had lying around. Both came as kits so I think they're common, and I've seen the buttons for sale in automotive departments. You are welcome to use whatever works for you and whatever fits. Plastic is easy to cut and drill and I've thrown up a version that has no holes in it so you can make your own.
optionally, some better JST connectors. In my case, I again used what I had lying around. Digikey part numbers 455-1648-ND, 455-1133-1-ND, and 455-1183-ND
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Assembly:
1. You can either follow the soldering instructions in the original Adafruit project or you can use my pictures as a guide. I soldered mine such that the single neopixel is the last in the chain rather than the first, and I included the capacitor and resistor because my neopixel rings will not work well without them (the singlets do, though, provided I'm not using 16 of them). The plastics are sized to be able to hide that awful capacitor so don't worry. Just make sure to get the polarity right. Also, make sure you've soldered to the Vout pad on the Gemma and NOT the 3Vo pad for power. (well, you can, but the colours will always be off. If you don't mind that, then you cause the 3Vo pad as it's regulated and hopefully safer)
2. Test your neopixel assembly before proceeding.
3. If using the switch, solder it up as in the pictures. If using alternative JSTs, then take the opportunity at this point to test and make sure everything turns on.
4. If using the button, solder it up and connect to the board. Again, you can see how I did mine in the pictures.
  For ease of assembly, use solid core wire for the attachment of the neopixels and stranded wire for everything else.
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  Or, you know...do what you want. :p
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  Not gonna lie...the electronics will take up probably a day or more, depending on your skill level. The plastics will only take a few minutes. The soldering is a bit tricky. None of it is surface mount, but you are soldering to an already-populated board, which can be tricky sometimes, and the final assembly benefits greatly from you cutting the wires the right length, which means working in tight quarters. You may be faced with a lot of wire splicing.
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  The battery can be expected to last about 1 - 1.5 hours, depending on how bright you make the lights. You'll know when the battery is dying as the colours will start to get warmer and eventually they'll flash strangely. I do not plan to make a version that fits a bigger battery unless I also make a version that fits a bigger neopixel ring. If you really want a big battery, then I suggest running a wire to your pocket and stuffing a really big battery in there so the weight isn't on your neck.
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  Oh, I almost forgot... I put some holes in it for string so you can wear it as a necklace. I thought of making a TPE necklace for it but really, yarn works just as well and is much faster.

Arc Reactor Sized to Neopixel Ring