Noow need!
I had a look and unless you have wood workshop, forget about it mate!
If you are brave you could use fiberglass, If you not concerned with stealf and have a k11, wack some holes into the shelf and strenthing up the underside.
Believe it or not the parcel shelf is quite strong and wont flex that much.
A word of advice, mount the 6x9 as far away from each other a poss help weight distrbution.