Mask Portion of Frame in FCP?

For a recent speaking event we recorded, I would like to remove (or cover) the audience portion of the frame from view. It's a bit tricky, though, as it's a split screen image, and while I want to remove/cover the audience in the lower portion of the left half of the frame, I also want to keep the lower portion of the right half of the frame, as the right half displays the speaker slides. I was thinking perhaps the best way was to create some sort of black overlay to cover the audience on of the left-hand of the frame, but I'm not exactly sure how best to do this in FCP. Any suggestions much appreciated. See attached screenshot for reference.