Recently I picked up a pair of Polk M7's at a local thrift shop for a little less than $30, and one of the speaker grills was broken. I decided to go ahead and fix the grill because other than that the speakers were in really good shape. IMHO, the repair turned out pretty well so I thought I would share how I went about it.
The first two pictures are of the grill before the repair, from both the front and the back. As you can see, there is a section of plastic missing from the top of the grill that needed to be replaced.