Trimming the rear holes to 6.5 I cut out the 4inch holes to thread into, just drill new holes to fit your speakers and use a fastener to tighten it down. I'm using some dynamat and locktight, the metal sure rattles a lot lol.
As for speakers. Went with some nice focal 6.5 components up front and infinity reference 6.5s coaxial in the back, I don't feel like mounting components that I won't hear back there. Digital designs 5 channel and a pos pioneer single din runs it all. Thats my system currently. Thinking I'll add a Rockford sub dual voice coil 2ohm at 1 ohm to give it some kick

Any questions feel free to ask.