Seeing as how the diffuser's well depth has no impact on the other dimensions of the unit and only changes the low frequency cutoff, I can change it to anything. What would you suggest?

Its application will primarily be for recording acoustic guitar and vocals. Speaking of which, I found this very promising quote from GIK Acoustics:
"The QRD really holds its own sonic imprint: warm, non-directional yet sonically cohesive. A great acoustic guitar played into a single quality QRD just feet away will sound like an even greater version of same guitar."
This coincides with what I heard from Ethan Winer's diffusion video and sets my mind at ease a bit with some issues raised in this thread. I suppose for most of my songs, I am primarily concerned with the acoustic guitar sound 400 hz and up. So, maybe if I dropped the well depth down to 8 inches?
You're statement about the polyurethane glue makes sense. Would you suggest Elmer's White? My only concern with this is that it is not 'supposed' to be used with non-porous material.
Thanks.