Time to shut down the project for now, it's raining too hard. The only thing left to do is glue in a set of magnets in place of the washers on the standoffs. Hot glue does not adequately bond to metal and the magnetic attraction was not strong enough to securely hold the grille. Good thing I bought 8 to begin with, for just this scenario!
So, I had my head up my . . .subwoofer, and I noticed a strong resonance around 80Hz +/-. I'm wondering, is this something I need to try and damp out, or is it just the Helmholtz resonant frequency for this volume. Depending on the correction applied for the neck of the cavity, I get Helmholtz values from 60-175Hz.
Lastly, a few progress pics:
Making the Grille - I used leftover spandex from when I made my screen
A few detail pics
And the final result
Once I get a decent break between thunderstorms, I'll mount the woof and see if my tuning frequency changed perceptibly with the memory foam lining. For transport to the man cave, I'll take the driver back out and carry that separately downstairs. This beast is heavy enough without the driver!