I designed these small envelopes to fit into the small (7.5 inch) lockable tin mail boxes available on websites like amazon. I use 3m Velcro to fasten it to my counter top. I place the envelope templates and repositionable glue sticks in a bin near the mailbox. Students can write me a note at any time. I check it, or have a student check it regularly. By using this type of glue, the envelopes can be opened without damaging them, and the teacher can read the note written on the inside.
This a great way to communicate with your students, especially if they have something to tell you, but they find it easier to write it down.