This is what I rigged up for my SPL meter:
The clear disc is a protector disc that you get when you buy a spindle of CD or DVD discs. The center hole of the CD disc is actually a little larger than the mic so I glued on a rubber washer that fits the mic more snugly (the diameter of the mic is approx 1/2 inch). It works and the clear disc is nice because you can see in to see where the mic is positioned.
That is identical to my rig--same SPL meter and using a junked CD. 
I don't allow the mic to stick out like that though, since I think having it flush with the CD is better. If the mic gets too close to the driver it'll skew the SPL reading to be louder than what you hear with headphones on. Remember, your ear canal is actually some distance away from the side of your face. If we rigged a short distance of tunnel with rolled up material to simulate the ear canal, and then attach the meter's mic to the end of it, we might get even closer (without actually buying an expensive dummy head for headphone testing).
Edited by Lunatique - 10/9/10 at 11:06pm
