[Tatjana Hamilton]"The only way I knew how to do it was using the letterbox effect. Worked it out in the end."
A better alternate workflow may have been to set up your project to match the aspect that you want your movie to be. Then render to the delivery aspect using a custom template that preserves the aspect ratio of your project. Letterboxing should never be done during the edit. Letterboxing is an artifact of mismatched delivery formats and should always be done during delivery/rendering.
For example: if you are working in HD and want your aspect to be 2.39:1 (Cinema Scope/21:9) set your FCP X project to 1920 x 803 and do all of your editing and cropping at this aspect. It will make editing a lot easier.
Then if you want to deliver that to 1920x1080 Letterboxed, just customize a template in Compressor and on the Video tab under Video Properties set the Frame Size to 1920x1080 and under Cropping & Padding set the Padding to Preserve Source Aspect Ratio and you will get your 2.39:1 aspect movie letterboxed in a 1920x1080 video.
This gives you the added benefit of being able to delver in your native aspect for viewing on a Cinema Scope TV or in theaters was well as being able letterbox for 4:3 SD delivery which you can't do easily if you letterbox in your project.