I made a custom pattern by wrapping my arm in saran wrap, then duct tape. I drew my details onto the tape while it was on my arm so everything would line up correctly, then carefully cut it off along the opening on the underside of my arm.
It helps to have a friend assist you with this part.
Copy the pattern onto a piece of paper and trace it out a few times to finalize your details.
Use the pattern and some aluminum foil to make a mockup of your forearm*. Get this as close as possible to the size and shape of your actual arm.
*Feel free to make a tin foil hat at the same time, nobody is looking.
Trace the pattern onto your craft foam. Be sure you flip the pattern so you have a left and a right.
Spray the outside of the foam with Plasti-dip to seal the foam and give it a nice leather-like texture. I found it more comfortable to leave the inside of the foam in its natural state.
Set the foam aside for now while it dries completely.
Peel the plastic off your acrylic and admire its perfection. Tape one of your neopixel strands along one edge. Then grab your favorite sharp object and make some scratches.
Really gouge the heck out of it. I mean really. Take out all your aggression. Scratch it some more in the other direction. Remember, the cracks are where the light gets in.
Use a piece of acrylic that covers MOST of your pattern but not all. The acrylic will be unyielding once it's shaped, so plan for it to cover just the top half of your arm rather than wrapping all the way around, or you won't be able to get your bracer on and off.
Place your Circuit Playground into the design and trace around it. With the scratches on the back side, cut out your shape with a rotary tool (or a laser cutter would work great here too). Remember to wear your safety glasses! This stuff gets everywhere. Polish up the edges so they're as smooth and shiny as possible.
Go find your aluminum foil arm. Pin some little aluminum foil feet to the bottom side to hold it up off the ground a bit and give more room for the acrylic to move and melt.
Line up your acrylic with the center line of your aluminum foil arm and balance it on a foil lined baking sheet in the oven. Sewing pins can help hold it in place. Bake for about 10-15 minutes at 300 degrees, until the acrylic is soft and pliable.
CAREFULLY shape the warm plastic with oven mitts until it fits your arm. Be patient - you may need to reheat it a couple of times and / or reform your aluminum foil arm to be closer to the shape of your actual arm.
Slightly too big is better than too small here. You'll have a layer between your arm and the acrylic so leave a little space.
Kevlar mechanic heat-resistant sleeves can help a lot in this stage. The plastic is hot! Be careful! Once it cools it will hold its shape very well.