I've started on a different approach. I've been doing lots of projects for myself with acrylic. I'm using a very thin piece of white diffusion plastic, (like for flashlight lenses and such) behind the triangle. I trimmed it to shape, a little oversized, and just loosely dropped it in behind the triangle window. It fills the area fine, but when you press the chest piece on, it always shifts just slightly due to the arc of the chest shape. Next I'll try a dab of non permanent putty to hold it in place. If that works, I'll light the chest up and see how it looks.
I'm hoping this works as a non balls out work around. You, Craig, have shown your balls out. (Wait, did I just say that? That's not what I meant.)
I *think* you are gonna find the black triangles I cut out (noted with the pencil) are touching the backside of the plate. You can 'cheat' and pull the chest plate out 1/8" to 1/4" and it will illuminate fairly well.