i wouldn't bother trying to fit a 'sub' onto the parcel tray. any speaker/sub requires very SOLID mounting to work effectively, and rattles can be a big issue. you can't attach a speaker to the parcel tray/metal without a lot of modification, and in the end, it's unlikely to sound great. cutting the metal is probably a no-no (in australia, a major mod like that would technically require an engineering certificate to approve the car remains structurally sound) but i'm unsure if the magnet would fit ok without that. you'd need a big slab of mdf wood or similar to hold the thing. then a cut-out in the trim. also, for infinite baffle to work properly, the boot becomes the enclosure, which means you need to seal off the boot from the cabin entirely, with only the back of the sub opening into it. a lot of work for possibly not a great results, which ends up modifying the car/trims (bad for resale).
a quality 8" or 10" sub may suffice, in a proper enclosure/subbox, mounted in the boot. the resultant sound is NOT going to be 'doof doof' UNLESS you tune the amplifier that way, to drown out the speakers. a properly tuned system blends the subbass in with the speakers to compliment the sound, not dominate.
if you want to go fancy, then have a part-fibreglass enclosure built into the side of the boot, following the contours of the carpet trim for maximum space utilisation. realise too, most shallow mount subs still require a standard size box (in terms of overall volume (cuft/litres)), though it can be shallow; but don't mistake a shallow sub as necessarily requiring a small box (i'm referring to non-inifinite baffle models). often not the case.
i'm unsure what brands are available to you, but perhaps a Digital Designs DD510 10" or DD1508a, or Image Dynamics ID8v3 or ID10v3. all are great subs that can work in small enclosures (<1cuft).
if you're really deadset on the parcel shelf option, for sure try it. will be interesting to hear how you go.