Basically what I did in my car was keep my speakers very close together on each side and I chose my crossover points and slopes based on the diameter of the speakers and their proximity. My 6.5" drivers play up to 1 kHz, well before they start beaming, the 2" drivers play up to 6 kHz, just up to the edge of beaming, and the 1" tweeters play the rest. By intentionally preventing beaming(technically, the tweeters will beam but let's ignore that) I'm not so concerned about how far off axis my ears are from any of the speakers, and the same applies to my passenger.
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My system: Clarion_CX305, Massive Audio Core-1, Cerwin Vega V600.4, CV V1500.1D, CV B2 150Wx2, CV Stroker ST65C active, CV V465 rear fill, CV VPRO124D 2.0 cu ft @ 36 Hz