Sometimes you want the effect produced by true W I D E  S C R E E N, wider than the 16:9 default of iMovie or most cameras (Filmic Pro, however, gives you many wide screen aspect choices). One way to do it is to film in 16:9, keeping the wider aspect in mind — you can even put painter’s tape across the top and bottom of your viewfinder screen to help you visualize the narrower height — and then do the actual crop in post.

Just use HandBrake. Window > Picture Settings > crop 96 pixels off the top and bottom (or whatever numbers work for you):

HandBrake letterbox crop