I made it at Techshop
This instructable was Runner Up in the Copy Cat Reverse Engineering Challenge.
Commercially available Grit Guards (http://www.gritguard.com) fit in the bottom of your car wash bucket and clean your wash mitt, brush, or towel, to save your paint from swirl marks and scratches. Once the grit is removed from your mitt, the guard acts as a settling filter, keeping the grit away from your mitt, so that you don't recontaminate it.
Grit Guards may be purchased for $9.99 + shipping and handling, and according to their website, is the "most cost effective method for protecting your precious painted surfaces from being scratched while washing your vehicle."
However, this instructable will show you how to make your own grit guard for FREE*, making THIS the most cost effective method to protect your vehicle!
If you must purchase one, Home Depot sells theirs for $2.60, which is still 1/4 of the price of a Grit Guard.
Tools (available at TechShop):
1" or 1-1/8" hole saw