The original question was from Audiohertz2, why you do not flush drivers, what is quite important for tweeter diffraction control, no matter what is crossover design or what is cabinet material MDF/HPS/Stone/CastAlu or how speaker performs in a different rooms.This is not presenting the reality its assuming and then drawing a line accordingly .
Fabricating something
I have measured Ls in may be 5 - 6 different rooms i measured ALL AROUND THE ROOM .
I design Ls as i think is suitable , you do/ measure as you please .
I use HPL by the way , not cheap MDF
A lot of musical energy will simply dissapate in your housing.
You should listen if you get a chance to Bass reproduction coming from a Braced 13 mm HPL cabinet , compared to your 18 mm MDF housing .
Different world altogether
The question was only about flush-mounting: https://www.erinsaudiocorner.com/driveunits/peerless_dx25tg59-04_tweeter/