Most car audio stores sell boxes for 6x9's. Usually they're charcol grey, covered in a carpet or felt. You can cut the box down to any size you want. In one of my old cars, i didn't have enough depth for the speakers. I bought boxes, cut the back completely off and used only the front face of the box. When i was finished, i had about 1/4" on the front and about 2-1/2" at the back, so the speakers were angled forward.
Then you tint the rear window so you can't see it from outside. Note that tint scratches really easily if its tinted before you start working in the rear window.
Good Luck.