Buy some thick mdf, cut out the shape of the speaker into the MDF.
Then get a peice of cardboard and make a template for each rear speaker and cut that out of the mdf. Then mount the MDF into the rear. It will reduce vibration and you can also cover the metal with some sound deadening.
If you want the cheap and easy way they just fit into the rear and can be screwed in place, you dont need to chop anything but they may be rattly as the metal is not that thick in the rear. They fit perfectly into the space and you can make them point slightly upwards. So 6x9's are easy in the rear.
