If you need some quick white balancing for whatever reason, and don’t have a white balance card or Expodisc handy, you can try using a standard coffee cup lid. Photographer Steve Bennet always has some lids lying around in his car, and found that they work as rough white balancing tools.
To use, I set the focus to infinity, place the cap over, then set the custom WB. I dont even need to hold it as it fits nicely inside my lens hood, but that is just a lucky coincidence.
Of course, this won’t deliver perfect results that rival professional tools, but if you’re not shooting RAW and need a quick approximation, you might want to give this a shot.