Along The Grain
When cutting the left and right pieces, it's important to orient the paper template so the corrugations are going along the length of the sides. This way the cardboard is a bit stronger and less likely to bend when cutting and handling.
The slots and cutouts for the tabs and components can be a little tricky. I found it easier to freehand cut than using a ruler. Slowly insert the tip of the blade and cut in a reciprocating manner. Doing this several times will cut through the internal corrugations. Flip the part over and follow the cut by slightly scoring the outline.
The Circuit Playground Express board can be secured to this cardboard piece using M3 sized standoffs, screws and hex nuts. To make the holes I used a Dremel rotary tool with a 1/8in drill bit. You could also use a hand awl (commonly used for making holes in fabric when sewing or stitching).
These M3 female/male brass standoffs are 10mm in height. I got a variety pack from amazon. These have a threaded hole on one side and a male threaded end on the other. Insert the male end into the cardboard and fasten a hex nut to secure it in place. Repeat this process for all four mounting holes.
Place the CPX over the four standoffs and orient the PCB so the mounting holes line up. Depending on your project, you'll want to orient the board so it's easy to access the JST and USB ports for connectivity. In my case I had the ports facing "right-side up". Insert and fasten 4x M3 machine screws through the top of the CPX going into the brass standoffs.
Install Micro Servo
Start by inserting the cable from the micro servo into the cutout. Then, carefully press the body of the servo through. It should have a tight fitting. Using the two mounting screws that came with the micro servo, insert and fasten them to secure the servo.
Before we assemble the rest of the case, we'll need to hook up the wires. Connect three mini alligator clips to VOUT, A1 and GND labeled pads on the Circuit Playground Express board. Grab the male jumper connectors and thread them through the bottom cutout in the center of the CPX. This will make it easier to connect the micro servo to the mini alligator clips.
Connect Micro Servo
Grab the male jumper cables from the alligator clips and connect them to the female jumper cables on the micro servo. Make sure the connections are correct. Yellow (siginal) to A1, Blue (ground) to GND, and red(voltage) to VOUT. Double check these connections are correct.
Start with one of the side pieces and the bottom piece. Insert the tabs into the slots and hold them together in place. While holding assembly in one hand, place the servo piece over in the center. Orient the piece so the tabs can insert into both the slots of the bottom and side pieces. Grab the cpx piece and insert the the tabs into the slot. Now the opposite side piece can be installed by fitting the tabs into the slots. Keep the orientation consistent with the opposite piece. Finally insert the last piece into the left and right sides.
Start by inserting the JST connector from the battery pack through the notch near the bottom piece. Thread the cable through the piece and connect it to the JST port on the Circuit Playground Express. Press the battery through the side piece and push it through the opening in the opposite piece. To keep the wiring inside, fold the wire into a bundle and secure with a twisty tie.