I used a Stick 'N' Click light (requires 3 AAA batteries), which you can find online not too expensive. (It's really bright; don't look at it directly!) Since I was copying the Avengers reactor, I used black duct tape and masking tape to mimic the lines and brightnesses of the reactor. Working with pieces of duct tape that small can be tricky, but don't settle with a piece until you're happy with it.
You can copy the way I taped my light, but I also looked up lots of photos and trailers for the Avengers. Sorry I didn't get any photos during the creation process. The masking tape is for the lines of the triangle that are a bit darker than the rest. No other duct tape color than black blocks the light enough to create dark lines. I would also recommend adding a layer of blue cellophane under the tape to get that blue color I missed.
To wear the reactor, make a sticky-side out loop of duct tape and stick the light to a shirt under the shirt you will wear over the light. The tighter this shirt is, the less slipping you'll have.
Enjoy being Tony Stark for a day!