I'd build a simple shelf using heavy duty L brackets. Install a small lip on the front of the shelf to prevent the speaker from spilling over the edge and then you can choose whatever material you want to prop up the back of the speaker so that it points downward properly.
If the shelf idea is too cumbersome, you could always attach three or four FMBs (or similar speaker mounts) to the back of the speaker and then attach those to the wall (to as many studs as possible). This should spread the weight out enough so as not to create too much sheering tension on any one screw. Oh, and use many screws to secure each speaker mount to the speaker (careful not to drill too deep into the cabinet).
