Before I got my Shackman lights, I used a jogging LED light that I picked up for less than $5 at The Sports Authority. It blinks in different patterns, has the same width and length (roughly) for the bottom chest slot as Shackman's lights, and can be easily duct-taped to the inside of the armor. You just need dark transparent acetate paper or limo tint to place over the slot, since the lights are fairly bright. For the top slot, I just put a piece of reflective red material I got from an auto parts store (which gave the illusion of generating light).
People loved the jogging LED lights when I first installed them, and I'm sure most people who see your costume (with the exception of TDH members) won't notice that it's not 100% accurate.
But, if you're a stickler for accuracy, Shackman's lights are probably one of the easiest modifications you can make. They can really turn a good costume into a fantastic costume.