I used one 10' 2 x 4 cut into two 5' pieces and fastened them to the rafters with 4" drywall type screws and this was for rafters running front to back, if yours run left to right you will need an additional cross piece. They say 2 x 6 in the instructions but what I installed is solid and ain't gonna go anywhere.
Got it. My rafters run side to side. They have drywall over them, so I’m assuming it’s safe to just mount them on top of the drywall and drill through to the stud with longer bolts to make up for the drywall thickness?