I'd buy another parcel shelf and cut it up, the diy mdf one will squeak and rattle like nothing else. Actually what I'd do is put them in the rear door cards, loadsa room, looks better and better acoustics.
Why do you want to make a lot of work for yourself, when there are perfectly good speaker locations in the front? (which will sound better)
Fair enough...The door grilles are cheap from VW, and it's pretty easy to feed the wires through. Just a thought.