Just make sure your mount can follow bend with the slpe of you ceiling and then just use the keystone to adjust it to square off. Usually it is best not to keystone because some dithering happens but because of your not so standard setup the keystone will work fine. Some projectors actually have no image change in keystoning but not all Panny L300. A lens shift can help too such as in a Sanyo PLV-Z1. As with all projectors ambient light control is a must to get a great picture.

http://www.quadloft.com/hometheater- new HT