I've created a template for anyone who wants to use it. It will allow you to use either 6.5" or 5.25" speakers in the factory locations. All you have to do is print out the template at 100% and use it to trace onto your material (MDF, plywood, plexiglass, etc).
The template hole was made to fit a 6.5 that had a large basket. If you are using 5.25" just cut the hole smaller to accommodate the smaller speaker.
The larger holes are for the factory mounting points/screws. The smaller holes are for the speaker mounting points. The speaker mounting points are just approximate as they worked for me, but you can pretty much put them wherever as long as they do not interfere with anything.
For mounting the speakers - If you use screws that are longer than the thickness of the adapter, they may run into the plastic behind the adapter. So try using smaller screws.
Download template PDF here