I was kind of inspired by the work in progress ported tweeter Erin showed on his chanel, which should make these designs a reality (while remaining relatively simple) if you use a 1.5KHz crossover (with steep slopes) you would need the centers to be 9 inch apart, which is kind of reasonable.
Then I tried to go the other way around with a small 4" midrange with really high excursion (Purifi PTT4.0X08-NFC-01). The Driver is 5 inch in total in diameter.
Extension, sensitivity and output are all remarkable for a 4L cabinet, the distancing for a typical 2.7 KHz crossover (5 inch distancing required) becomes a little bit of a nightmare as you're looking at only 0.4 inch of wood between drivers.
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The tweeter low frequency extension and output seems to be the biggest limitation, and when you job to a compression driver with a waveguide you're already way above the distancing requirements. The ported tweeter is aimed at car audio, but i really think decent horizontal centers would also be a great application.