I started with the Think Geek arc reactor. I found an image of the new arc reactor and scaled it to the size of the one I already had.
The triangular section was printed and cut out in card stock.
I traced the design onto some plastic I had from a Thai food container.
The plastic from the old piece was trimmed and the new plastic was inserted behind it.
Assembled, no lights.
Hooray! No more of that pesky palladium poisoning. It's certainly not perfect. The LEDs are spread out to accommodate the old piece, but there's a decent amount of light diffusing through the white portion to make it look good.