If you want to dim the light and don't need to easily crank it back to full brightness, a non permanent solution is to put a piece of theatrical color filter inside the cover. Roscolux is a popular brand. It comes in 20" X24" sheets, can be easily cut to size, and stands up to heat (it is designed to go in front of a 1000 watt theatrical fixture).
A simple frost such as R116 will diffuse & soften the light, while one of the neutral density filters such as R98 will cut down the light by 75%. They have other ND filters that cut it less.
Here is their catalog. Check page 12 for the grey. The material is available at Amazon, or a local theatrical or good photo supply store. A theatrical supplier might give you a "swatch" book, a 2"X3" sample of each color. I believe Amazon charges for one.
