5 M3x10 screws and nuts
14 White 5mm LEDs
10 Blue 5mm LEDs (optional)
Resistors (Depends on your configuration)
Print the crystal in clear PLA and the other parts in a dark color, metal or black are best. For best result print the crystal with a low rectilinear infill and no more than two shells.
Assemble everything except the base. I put some aluminum tape on the underside of everything that I did not want to be translucent, but this is optional.
Put white LEDs in holes that are not covered from above by the top piece (10 around the outside and four in the middle), optionally put blue LEDs in the other holes to give a nice bluish glow to the outer ring. I sanded the LEDs before wiring them in to give them a more diffused glow.
Wiring of the LEDs will depend on your particular LEDs and the power source you use. I am running off a 9v battery and have eight parallel rungs of 3 LEDs in series with a 68 Ohm resistor. My LEDs are a little under-powered and could be brighter with smaller resistors or if they were wired two in series instead of three, but they are bright enough. You can use this calculator to design your own circuit: http://led.linear1.org/led.wiz
Cut a hole in a t-shirt just big enough to fit the crystal, pin the edge of the cloth in the groove between the top and base pieces and secure with screws. Attach some sort of belt using the slots in the base to go around your chest so the arc reactor does not move around.
You will need to run a wire under your clothes down to your pocket to your power supply as there is no room to build a battery into the reactor itself.
P.S. I strongly recommend not trying to get on a plane with this on.