You have footage from a client that's green screen and need something…anything for the background right now!
1. Find a still image, really just about anything and bring it into FCP.
2. Drop it in under the green screen person that you've already keyed.
3. Add Gaussian Blur - try 20 or so.
4. Add "Stripes" which is here: Video Filters > Distort > Stripes
5. Monkey with the Offset.
Try various levels of blur and different still images or frame grabs from the same project (just don't let the client see you do this :) (It's Shift + N, btw)
You can use video clips as well but they may take a little bit to render. Try slowing them down and/or key-framing the Offset parameter.