Just block the speaker off the wall to suit your build depth. You are thinking a 2D skyline. Didnt do one of those. More of a ceiling thing.

I used EPS for my Arqen fractal upstairs. Built a hot wire foam cutter and worked well. Pretty accurate cuts and was a fun project.

For the Big QRD downstairs I used 1/8” wall panelling cut into strips for well dividers and 3” wide rigid packing foam. It is over 6’ wide and 48” tall or so.

Gotta be a rigid product whatever you use. The arqen is way lighter than a true QRD. Sections can be held in place with 3M command strips… but not a true diffuser like a QRD.