NOTE: If you want to buy the rear MDF panels from me to fit larger speakers in the back, I can custom make them for you for $50 + shipping. They will include the panel itself with a hole custom cut out for 5.25, or 6.5/6.75" speakers, 10 short black wood screws for mounting the speakers and 10 self tapping high quality metal screws to mount it. Just comment on this post if you have any questions or would like to order a set. They work on all FRS's and BRZ's.
The last person that ordered them from me was worried about all the sound coming through the 4" hole, but you actually don't need to worry about that at all because the MDF plate holds the speaker about 2 inches away from the hole, so all the sound is able to come through with no loss.