If you truely want to "knock down" some of the light, meaning reduce the intensity while preserving the quality (ie hard), then what you need are scrims. These are made of wire and come in single (half stop) and double (one stop) designations and are designed to be used right next to the lamp. There probably is a slot in which several will fit.
Sometimes you'll want to use a single and a double or even two doubles. Scrims are also available in "half" designations, meaning that the scrim only covers half the opening, allowing for diminished level on top, bottom or side, depending on how the half scrim is oriented in the light.
Now, tough spun is a horse of a different color! It is also designed to accomodate the high heat of an open faced quartz light, like the Redhead you describe, but it's intended use is to "diffuse" or soften and spread the quality of the lighting instrument. With its use the shaddow will become softer, and yes the output will be slightly dimished. In addition to spun, there are other types of white diffussion such as Lee 216, 250, etc. which have various degrees of softening effect with the concurrent effect of using up some of the lights' intensity.
If I were you, I'd invest in a scrim set, of a single, double, half single and half double for every open face and fresnel light that you own, then also some diffussion gel to use to create various softening effects. There is even white diffussion that's like shower curtain, and while it is not as heat proof as the spun of other materials intended to mount right on the barndoors, when used as a panel in front of the light can create a beautiful soft source. In addition an open faced quartz light makes an ideal bounce source ito a white card of foamcore, you just have to consider controling the spill of off the walls and other objects.