Place the Circuit Playground inside the base, with the USB port facing the smaller hole nad the battery connector lined up with the larger hole. Secure it with a little dab of hot glue on the case's pillars.
Set the 3d printed button pieces aside for a minute.
Place the cover into the top and tape it in place from the outside. Screw the top onto the base. Once the top is tight, adjust the cover's orientation so the holes line up perfectly with the neopixels and buttons.
Unscrew the top, being careful to keep the orientation right, and glue the cover and top together securely.
Turn the lid upside down and set the buttons small-side-down into the button holes. Screw or press the base with the Circuit Playground inside into place so the neopixels are aligned and the buttons contact the Circuit Playground's buttons.
My 3d printed threads didn't hold very well, so I secured the top with hot glue.
Plug your battery or USB cable in and make sure the lights are lined up perfectly through the holes and the buttons are satisfyingly clicky.
The left button will cycle through color modes and the right button will turn the LEDs off.
Warning! this button does not turn the Circuit Playground board off entirely, so if you're running from a battery, keeping it semi-off like this will drain your battery eventually -- unplug your battery if you want it off completely.
The holes are sized to snugly fit 2.75mm light pipe. The best source I've found for this is these cheesy light up shoelaces (available everywhere!)
With your LEDs on full brightness these will glow for up to 3 feet, though the light does get quite a bit dimmer toward the end. I find they're most satisfying if kept to a length of 1-1.5 feet.
Or you can use fiber optics. There are two different kinds: end glow and side glow. The side glow are my favorite since they glow along their entire length. End glow are useful if you just want a single floating point of light at the end of each strand, also a beautiful effect.
My favorite source for fiber optic cable is Weidamark lighting. They've got loads of options and sell the fibers by the foot. (Note: fiber optic solid core side glow appears to be the same stuff as the LED shoelace light pipe mentioned above). Order in any size bundles; you can easily separate them down to the right size bunches.
I can fit about 10 fibers in each 3d printed hole, so if you use all 10 holes you'll want around 100 fibers. Like the light pipe, these seem to be brightest and cheeriest in the first 1.5 feet or so. After that they do still glow but get a little dimmer. If you need more length than that, I'd recommend going with end-glow.
Trim the fibers sharply and cleanly with a pair of snips or a hot knife. The cleaner the cut, the brighter the fiber will be. Start by slipping around 8 fibers into the hole, and then keep adding fibers to the middle of the bundle until they're nice and snug.
If this will be going on a costume or art car, or something that will move a lot, it doesn't hurt to add some extra hot glue around the outside of the fiber holes to secure them.
DO NOT USE SUPERGLUE HERE. Most superglues will degrade the fiber optic cable and it'll stop glowing and break. Sad Christmas.