By not isolating the back wave from the front and sealing those gaps up, you're going to have a very bad reduction in the quality of reproduction these are able to achieve. You should either pick up a 6x8 to 6.5" adapter or cut one yourself so you can mount those up properly. I suspect you will...
I know the thread is old, but I enjoy your build log. Can you update with pics of the finished work? I'm interested to see how you finished out the sub box and cover.
I'm looking for a pair of stock interior plastic panels for the driver and passenger side A pillars to fit my 2013 Mazdaspeed 3. I know I can go to the dealer for these as well but Im hoping to source some here to save some money. Perhaps someone has swapped theirs out for guage pillars, etc...