Elegant way to present 4:3 material in 16:9?

I have this realy old master of a documentary shot on U-matic (4:3 a.r.) and now I would like to thro it into a 16:9 format with subtitles. Maybe it's an idea to put this footage in an overlapping mask, also to 'mask' some distortion of the original material. But how do I place this (black) mask as a frame/passe-partout?
Suggestions? (yes, I am only basically familiar with Premiere Pro).