The cutout in the blue piece is to avoid the motor for the power windows.
I glued the two halves together using the yellow 3M weatherstripping glue, plus some rubber tape I had lying around. I also added some foam baffling I had - I wasn't sure if the plastic would vibrate or add any sort of weird sound to the speakers (I'm definitely not any sort of audio engineer...!!)
Here it is in place with the first of the screw clips in place. I had to drill some new holes to mount it.
This is the new speaker - a 5.25" Infinity.
The end result... I'm very happy with the fit and sound!
Let me know if anyone's interested in getting the STL files to print their own set - I'll see if I can figure out how to share...