I haven't had the car apart but my concern would be
(1) Supporting the weight of a heavy magnet (sub driver) and
(2) Noises, or rattles likely to be introduced via the parcel shelf design and mounting characteristics.
I also think the driver would be too far from reflective surfaces to yeild the proper sound waves for good accurate low end.
The box is specifically designed for accoutics, size and air volume and to be 'sound dead'.
I would not think of the rear shelf as the last option for a sub. If I could not set one up in the trunk, I'd go without.
There are some reputable name brand thin-mount designs out that fit along wheel wells or behind a seat like the Jeep Wangler if concerned about trunk - luggage space.
'04 G35 6MT, DG Coupe, Prem/Nav, purchased @97k mi