2iemniak posted a fantastic free model of the Iron Man armor which is available at http://www.blendswap.com/3D-models/characters/iron-man-mk3/ . I pulled out the arc reactor pieces and tweaked them for printing.

Pull up a BOX OF SCRAPS and enjoy!

I recommend printing the core out of glow plastic, if you have it. The shell that holds all the pieces will likely need to be modified depending on if you want to put it on your desk (print with supports to reduce mess) or wear it (ditch one of the layers so it can lay flush on a t-shirt.)

UPDATE: Added two files that can replace the "shell" if you'd like a sleeker reactor to wear on a shirt. Attach magnets to each piece and you'll have a prop that can "split" and grip the fabric of a t-shirt.