The picture below shows all the parts, ready for assembly. The STL files for printing the case are at the bottom of this page.
We'll start by mounting the LiPo backpack on the ItsyBitsy using the long header pins provided. Remember to trim the header pins afterward.
In order to use the switch for power, the trace between the switch pads must be cut. That's shown here.
The next step is to add wires for the NeoPixels, buzzer, and rotary encoder. I used yellow for these. Be sure to make them plenty long, you can always trim them later. 120cm (4-5 inches) is fine.
Next add two wires for the power switch. I used green for these. Again, 120cm or so (4-5 inches) is plenty.
The last step for the ItsyBitsy is ground and power. The ground hole is being used by the LiPo backpack, so you will need to solder the ground wire onto the end on the pin on the backpack.
The next step requires a fine drill bit. I have a small jewelers drill that's idea for this, and a drill bit that fits a through-hole PCB hole. Position the NeoPixel ring in its place such that the Data-out connection is at the bottom. Drill a hole in the case at the Data-In, one power, and both ground holes.
Feed the power, ground, and Neopixel connection (from D11 of the ItsyBitsy) through their respective holes you just drilled in the case and solder them to appropriate holes in the ring. Verify that it's soldered on both sides just in case the plated holes were compromised during the marking/drilling (it's unlikely but you can't be too safe). Cut another short (around 60cm or 2-3 inches) length of black wire and feed that through the second ground hole in the case. Solder that to the second ground connection on the ring.
Now gently pull the wires through the front case and settle the ring into the circular groove. A few dabs of superglue will help it stay in place.
Next use a bit of superglue to hold the buzzer in place. There is a ring inside the top part of the case to help get it in the right spot.
Next we can wire the rotary encoder. Start by stripping/tinning about 1 cm (0.5 inches) from the end of the ground wire coming from the ring. Connect this to one side of the encoders switch (the side with two connections) and the middle connection (of 3) on the other side.
Connect the correct wire from the ItsyBitsy's digital pins to the encoders other pins (see the wiring diagram). I used short pieces of heatshrink on the connections to strengthen them as well as avoid any chance of shorting. Always a good idea. Wire the two green wires from the LiPo backpack to the center pin and one end pin of the slide switch. Which pins they connect to doesn't matter. I generally clip one of the end pins off. You can see this in the photos below.
Connect the final wire from the ItsyBitsy (from D12) to the buzzer. It doesn't matter which connection. Cut a piece of black wire to reach from the ground connection on the encoder to the other connection of the buzzer. Again, I used some heatshrink on these connections.
Now we can use superglue to mount the ItsyBitsy and the power switch.
All that's left is to connect a battery and seal up the case (careful not to catch any of the wiring between the parts of the case. My case design is snug so it might take a little finessing. I used a few dabs of superglue to hold the two pieces together. It could be redesigned with a fancier clip system if desired.
Here are the STL design files for the case for you to use to print a case (or sent out to be printed):