Well to answer WTH was I thinking is crawling around in the trunk is a PITA so I am NOT lazy and I have owned it since 1985 (yes my high school hot rod) I have spent the last 12 years putting this car back together.
The seat back does not have an area for all 4 mounting points and I don't want to hang it by just 2, yes I can make a bracket for the other 2 mounting points but again it is PITA to crawl around weld anything in the trunk. As far as mounting it under the package tray I have bee told to NEVER mount an amp upside down due to the way they exchange heat and mounting it upside down will cause failure. And before you ask about the flat upper area in the trunk floor that is not an option due to a mini tub kit and the cross member support that runs across it.
I have seen amps mounted in this location but it seams like the heat would be an issue, but I have also been told that the amp creates so much heat that any heat from the rear window is insignificant in comparison. I found that hard to believe so I am reaching out the experts and fellow Camaro enthusiasts.
What I have done is reach into a trunk with a tape measure and see how tall and wide of a board I can cut to attach to the existing metal on the car and when doing this have this board larger than the footprint of the amplifier.
Now I put those measurements onto a piece of 1/2" or 3/4" plywood and cut it to the dimensions I need.
I then reach into the trunk and put the board up against the metal and see where I can drill through it to where I can have 4 or more screws holding the board to the metal behind the seat.
I then take the board out and drill the 4 or more board mounting through holes into the board to allow the screws to pass through them and push through these holes.
I then take the board out of the car and lay the amp on the board and mark and predrill holes for screws to mount the amp to the board but do not attach it as yet as some of the screws to attach the board to the car may be covered.
To give the board and install a more finished looking, I sand the edges of the board and paint it.
Once it's dry then I climb into the trunk to hold the board where it needs to be and then attach the board with screws to the metal behind the seat. Depending on the screws, I might have to mark and predill holes in the metal or may be able to use self tap screws to make their own holes.
Once the board is mounted, I make all of the electrical connections to the amp as well as setting the gains or any switches on it to the proper spot and then place the amp in position to allow me to put in the four screws into the predrilled holes in the board to hold the amp solidly to the board.
Once this is done, I grab some wire ties and detail the wiring and cover it with split loom tubing to finish it off.
I have also, in some installations, taken a board larger than the footprint of the amplifier and actually found a decently flat spot on a trunk floor or on the back inside cab wall of a truck for example and then attached the board with liquid nail and let it dry a few days and then mount my amp to it. If later the game plan changes I can pry the board off, remove the glue and not have to worry about holes in the metal surface.