Ive used a custom made 3x5 card in the past.
https://www.dropbox.com/s/pj7jrzluos...cord%203x5.pdf

However since the recent update of the Reciprocity Timer iOS app I may just use that now. Ive used the app for a long time to calculate my exposure but the update lets you save all that info into a note where you can add additional info and even a photo that you can mark with where you metered for the zone system.