Part of a project in another forum investigating the vintage EV "Baronet" mini corner horn, I closed the rear and installed "modular" JBL acoustic suspension two-ways.
Best I've been able to locate in the specs, 8" in-walls want 1.4 - 2.8 cuft loading volume, while 6.5" versions are designed for 0.7 - 1.4 cuft. Baronet is 1.5, typical bookshelf size.
In-walls are inherently engineered closed-box designs, and with applicability in HT installs, performance is garnering more attention. JBL has offerings in Pro Control series, Harman Consumer, and Synthesis product lines. These SP8s have Ti-laminate woofers and swiveling Ti-laminate tweeters.
Red is corner loaded horn, open back.
Green is closed box, in freespace, i.e., no boundaries.
Cyan is closed box in the same corner, providing more extended bass, lowered 80 Hz boom. 27 Hz - 17.5 kHz (-10 db).
[Yes, I should be using sinusoidal. Working on that.... ]