I had to cut a new bracket to fit a 6.5 in speaker. I traced the original face that the speaker mounted to.. then searched the house for something circle and the same size of the speaker. luckily I found a small sauce pot that work out great.

To avoid having the bracket from being seen through the rear window I ran a sharpie over the wood to black it out.

Out with the old..

On the bottom side that will be sitting on the rear deck lid I placed some double sided car moulding tape on the back to get a good contact.