Using a acrylic shield as a filter to "fix" the spectrum is interesting. You could use a gel filter from http://www.rosco.com/filters/roscolux.cfm
to reduce the yellow-green leaving a higher percentage of the far red to near UV colors. You would need to use brighter LEDs too to make up for the loss. I hope someone experiments with this.
That's an even better idea. Might have to give that a whirl, looks like there's a dealer quite close to me. My fixture might be lacking the punch to get the most out of trying this but it's seems like it could be a great idea to play with as far a tweaking my current spectrum goes.
Thanks for the link hoppy.