The spacer is strictly for clearance. Of course the larger the spacer, the more its going to interfere with the e-brake and dimmer switch. If you want to get to audiophile standards, ABS plastic panel and a metal backing isn't the best scenario for an enclosure
From what I can tell, and I can do some measuring this evening, anything less than 2" should work without a spacer. Of course you can get these enclosures loaded with a speaker that is guaranteed to fit. I also see a lot of guys online using a Kicker coaxial.
Given that I can get Rockford at dealer cost and I really want components I'm trying to make a 2.24" deep speaker fit without "massaging" the kick panel. Hart_Rod's speakers are about a 1/4" deeper than the ones in my pics.
I agree with sound deadening material on the metal kick panel and plan on doing something like that myself.