In my old Bianca ph1 I took the stock speakers out the shelf and removed the grilles, then cut a piece of MDF and marked up the speaker holes inc holes for tweeters and cut them out slightly smaller than the outside of the basket for the main drivers so I could router out a lip and the speaker basket would sit flush in the hole and secured them with 3 screws as my infinity kappa component drivers had various fixing holes. It just so happened that the other holes lined up perfectly with the holes to secure the grilles.
I also had to make the speaker holes slightly bigger on the original shelf to accommodate the slightly larger infinity speakers but did that with a rasp file.
I then fitted the tweets but cant remember if it was to the factory grilles or the shelf itself and screwed through the MDF into the original grille holes to secure the speaker and grille sandwich.
Once I was happy with the way everything fitted together I partially stripped it again and applied copious amounts of liquid nails to scuffed up MDF and shelf surfaces and quickly screwed it all back together, then added more screws to strategic places where the MDF met the shelf for added strength and rigidity.
This was all done on a ph1 parcel shelf and from the outside you couldn't see - totally stealth.
Sadly I blew those comps by under powering them and they were the old style green weave coned CS series
that are so much better than a lot of the ones they sell today.
I guess I'm saying it can be done without going down the ph3 stealth shelf route which can look so obviously out of place on an earlier car - even to the opportunist thief who knows what a stealth shelf looks like and might imply.
I have also been down the genuine Acoustic Solutions genuine stealth shelf route with a pair of 6x9's and that was sooo heavy the tailgate never stayed up properly and I kept banging my head so that had to go.
Now I've got a ph2 I bought a stealth shelf myself with the sole intention of removing the speakers and mounting some v v quiet fans in there place to remove the heat from 3 Alpine V12 amps and a car Pc install I have planned for the boot in the new year but if the fabric doesn't flow enough air I'll be going back to a ph2 shelf and grilles.