Maybe I'm nitpicking, but if your sub doesn't fit without a .5" spacer ring, I suspect that you have less than 1" behind the pole vent. You need at least 1" behind the pole vent to get adequate cooling. Otherwise, you can overheat the coil (aka fry it) in a high power application. Then you'll be looking at a $250+ repair bill.
JL rates the power handling of their subs at exactly what they can handle. If you approach their published limit, the pole vent has to be unobstructed.
You can reduce the internal volume with some styrofoam.
2013 F30 328i - Melbourne Red
2011 E90 M3 - Monte Carlo Blue
2004 E46 M3 - Imola red