MM120's in an Opel GT

Systems

I want to install a single 12 inch Polk Momo sub under my parcel tray in my Opel GT. Unfortunately there is only 8.5 inches of height in between the parcel tray and the gas tank cover. I have 3 feet to work with side to side and 22 inches of depth, so I easily have enough space to have the proper internal volume. The problem is the speaker would be pointing almost straight into the parcel tray. The parcel tray is pretty flimsy steel (maybe 18 gauge) and I am afraid it is going to really rattle. Will dynamat or some other acoustic dampening take car of this, or does this location sound like a bad idea? My other possibility is to make a traditionally shaped enclosure which is narrow enough to fit under my parcel tray, that I can take out. There is almost zero cargo space, so I need to be able to tuck the sub away, and preferably be able to use it tucked away. I do not want to cut the parcel tray, because they are rare and expensive to replace. I was also wondering if the speaker volume should be added to the .88 cu ft enclosure volume, or if it is included in that size (if so what is the speakers volume)? How important is it to be close to perfectly accurate in the total speaker volume? If anyone has any clever ideas for installing in an Opel Gt please let me know. Thanks.