Update:
Ok, bought a piece of 12mm thick MDF from b&q, for £6
Gave the guy dimensions, and he cut it for me.
I measured it up, so that the MDF just fits inside of the back of the existing parcel shelf (pic below).
http://erc.qmuc.ac.uk/cliosport/gallery/full/1107817790__MDF.jpg
What this means is, I still can use the original shelf with hinges & strings, and the original shelf hides the MDF.
As for the MDF, it actually sits on the same sills as the shelf (the sills have an extra cm on either side, which means both MDF and shelf are supported by them).
I have already used 3nuts/bolts/washers to secure the MDF to the shelf. The speakers and AMP will add more bolts & rigidity to the structure, so theres no need for more.
I will not go for "stealth" because accoustic carpet is far too expensive for what it is. I will go for this look:
http://erc.qmuc.ac.uk/cliosport/gallery/full/1107817829__Shelf1.jpg