Some systems require you to put a plate behind the headliner to create a "headliner sandwich" with the plate and the screen unit. I seriously doubt most manufacturers want you to screw into a cross member of the roof, that really limits where you can put the screen. I know the 20 or so I've put in for friends and family all had some sort of plate system to screw into, for the most part. There are other systems out there, and I can see how screwing into a cross member would make it much more stable, but again your placement options are limited. Really read those directions well, and if you are uneasy go seek professional help before you completely screw up your headliner.