I cut a window out of a piece of polycarbonate to replace the IR-block filter in the modified Canon S300. It's 1.0mm thick, and the infrared block filter was 2mm, so it didn't solve my focus issues. But it helped. I realized today all I need to do is cut another one out and 1+1=2. Sure it won't be optical cement-glued together, or anti-reflective coated, but who cares. I cut the polycarbonate (which is really the protective cover of a package of CD-Rs) with a dremel and ground the sides to fit.
The windows. The cyan one is the original.
This blurry photo is an example. I include it because of the delightful color variations from the mixed lighting. Fluorescents around the periphery and metal halide in the center. The carpet in the center is nearly black to the eye.
UPDATE: Gah. Adding two windows pushes the CCD back too far, resulting in out of focus images.