how much airspace do midrange drivers need?

[The Camry]

I've seen the a-pillar PVC endcaps way of running 3 ways. Just curious on if that really would be enough airspace for one.

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[trumpet 5,000+ posts]

It depends on the driver and the intended passband in which you want to operate the driver. For example, CDT recommends mounting their ES-02 and HD-2 2" mid/tweeters into a 2" PVC endcap if you will run them down to 250 Hz, which is the published low end of the frequency response range. You would not want to use an enclosure smaller than that if you expected good results down to 250 Hz. This is the same concept as designing a subwoofer enclosure.