Using a QRD to do absorption is beyond what we've discussed up till now. The
RPG abfusor does this using a porus floor and rough walls, but the science is their trade secret. Certainly a porus floor will change the effect of the diffusion.
With the knowledge we are likely to come up with, you might be better to do a normal QRD diffuser flanked by absorption panels.
Your new found depth of up to 2 ft allows you to make the design frequency quite low, however I would be wary of going to 25mm wide wells if you use the maximum depth.
AFIK Xenon's build with the blue foam was more for ease of construction than for absorption, but could be wrong there..
I'm happy to be a code wrangler, but leave the application to those who have some experience in these matters.
Perhaps Terry, Lupo, Xenon or some of the trade pro's might chime in....