Another option is to mount the speakers on the side walls pointing down.
If possible, that would be my first choice too. In my experience, the "rear" speakers in a 5.1 system are not really for the rear... they're for the sides. The sounds sent to them in a show/movie are sounds that are supposed to be happening to the side of the listener. For sounds that are supposed to be happening behind the listener, you can go to a 7.1 system. But honestly, the rears in my 7.2 see very little use. Most stuff is only in 5.1, and stuff that is in 7.1 has very little happening behind you.