Cut a piece of gaffer's tape to the length of your pixels and about 3/4" wide. Center your pixels on the tape.
Tape the pixels securely to both sides of your acrylic with the wires coming out the bottom end (the end toward your elbow).
Temporarily tape the Circuit Playground in place with the USB port pointing at your elbow and the battery port pointing at your wrist. Feed the wires along the inside of the bracer, flush with the plastic, until they comfortably reach the VBAT, 12, 6 and G pads. When you find the appropriate wire length, trim the wires.
Grab your loose neopixels and measure their wire lengths the same way. Then, twist all 3 red wires together and solder into VBAT. Do the same with the black wires into G.
Connect the data wires from the neopixel strips to pin 6, and the data wire from the individual pixels to pin 12.
Glue the individual pixels face-out against the acrylic in an artistically random arrangement. These pixels will provide the "sparks" from the imaginary bullets you're blocking.
Plug your battery in and shake the bracer around a little, making sure everything works.
If you don't want your wires showing, add some black tape or contact paper over them on the inside of the acrylic.
Cut two strips of Worbla to the length of your bracer and about 1" wide. Cut a slit in one of the strips to accommodate your on/off switch.
Hot-glue your switch to the outside edge of your acrylic and mold the Worbla over it to hold it in place. Hide the battery wire inside the Worbla edging.
Place the other strip of Worbla over the neopixels on the opposite edge.
Cut a circle of Worbla to create a mount / cradle for the Circuit Playground. Be sure to leave space for the wires and the battery cable to pass through.
With the Circuit Playground tilted slightly so the battery cable can pass on the inside of the gauntlets, and the USB port is accessible from the outside, gently melt the Worbla in place around the Circuit Playground.