I designed a mirror assembly so I could have the projector sitting flat with the image shining downwards.
I created a mirror assembly with a first surface mirror that allows me to adjust the position of the mirror when aligning the projection.
I dado'd slots into some plywood. I could have used a router table for this, but I personally feel much safer, and therefore more precise doing dado's on the table saw (it has a Saw Stop), rather than the router table.
I used long bolts and wing nuts to attach the assembly to the crown, which gives the ability to adjust the reflection angle.