Second that. What counts is a personal gesture. A plain white card will do. Write it by hand and put an envelope around it. Don't worry about the text too much.

Second that. What counts is a personal gesture. A plain white card will do. Write it by hand and put an envelope around it. Don't worry about the text too much. Make it short and respectful. And don't try to be too original, which might come across as intrusive.