Right, i should have done the guide a month ago when i fitted mine.
I removed the rear seats, unziped the back of the seat. You will see the metal grid.
With a knife, i removed some of the foam material where needed. About 1cm in depth. (the foam is not completely flat, it has some curves etc. I cutted down the excesive foam)
Stack 2 lenghts of MDF in between the foam and the steel grid where the amp screws would go.
Obviously this way the amp and MDF will hug the steel grid and will sit nice and flat with the upholstery.
Too bad i didn't get pictures at the same time as i added a cigarette lighter in the boot and a second boot light.