Looks like an X9 to me, also. It looks like it provides structural support - that's a pretty thick piece of material.
Post pics of the finished product. I have a 12" mounted in the panel that's just above that footrest.
You have the seat out - get down and stick your head up in there and take a look see.
. Or remove the front seat cushion, I think you can get a look from the backside.