[TP2SS]

I was going to guess under that rear compartment under the wireless charger pad of the console, but I'm guessing you tried it in there and it doesn't fit.
GM parts diagrams are scattered with references to "sound absorbers" that do not show exact shapes, so that's why I said "sound deadener" earlier. GM does not use "Dynamat" that I've seen. They use these foam fillers in many places, as well as "jute". Jute is the thick padding stuff under the carpets. I know because I had an Equinox that got some water at the floor level when an airport parking lot flooded some years ago where my car was parked. It barely got up to the door sills, but apparently enough to leak and soak the F&R floor area. The fix determined by the dealer/insurance was to replace the "jute" and clean the carpets.