I built an "arc reactor" out of an exploded LED flashlight and a plastic bottle a few years back, but for a while I've been thinking that if the light source was electroluminescent, the light distribution would be more even and it wouldn't stick out of my chest so obviously. I've worked with EL wire before, and when I saw that cutting EL panels into custom shapes was even more difficult, I decided to take an easier route.
Get a 10x10cm EL panel and an inverter to drive it.
The overlay is only half a millimeter thick to keep it about as flexible as the EL panel is. I've found that black ABS parts only a layer or two thick are completely opaque, whereas thicker parts printed in other colors still let some light through. Print this part in the most opaque plastic you have.
The tabs on the backside have no support, so you'll have to separate them with a blade. They're not as springy as I hoped, but they hold the panel in just fine.
Run a velcro or elastic strap through the sides and start wearing it!
I've added a blank version if you want to make a different generation arc reactor.