For more info on fitting trailers inside flat and scope containers in Easydcp Creator, see this post
Trailer DCPs should be delivered in two resolutions: 1998×1080 (Flat) and 2048×858 (Scope). This way the cinema can play the trailer before both Scope and Flat movies without changing preset on the projector. With common height cinemas, a preset change involves zooming the lens and changing the side masking.
For a Scope movie, you should deliver the Scope trailer filling the Scope resolution 2048×858, and then use “fit to width” in a Flat 1998×1080 project in After Effects or similar software to make the Flat trailer.
For a Flat movie, you deliver the Flat trailer filling the Flat resolution 1998×1080, and then use fit to height in a Scope 2048×858 project in after effects or similar software to make the Scope trailer.
Sometimes I see DCP trailers for Scope movies that do not fit the presets. The flat trailer is correct, but it seems they used this flat trailer to make the scope trailer. Instead of fitting it to the width, they fitted it to the height of the scope resolution 2048×858. This results in a scope trailer with a small letterboxed and pillarboxed picture.
A Scope trailer without soft subtitles could be shown at both flat and Scope presets. When played with a Flat preset only the pixels between 1998 and 2048 would be cropped. A local distributor could add normal 8/14 “from bottom” subtitles to make the scope trailer and add 22/28 “from bottom” subtitles to make the flat trailer.
Trailers should have a good cinema 5.1 sound mix. Trailers are normally louder than feature films and the cinema may play the trailers at a lower volume setting than the feature films.
If making commercials/PSAs that will play together with trailers at a cinema, you should try to match the sound level of trailers and deliver in both scope and flat formats.
To deliver trailers you could use a service like filemail.com business which lets the cinemas download the trailers as zip files. Filemail lets you track the downloads and the download speed is fast. You can also send the trailer on USB flash drives.