Positive Affirmations; not a new concept but it seems it is one that is not a focal point.
Make positive comments front and centre there is no reason why these should be said and then left alone. How about share with the class; the school; the parents.

I came across this post by a fellow Pre-Service Teacher Kimberly Dodd; titled: Creating Positive and Inclusive Environments: Positive Affirmation/Quote Wall

I love the idea of a positive affirmation wall!

Instead of affirmations being shared between 2 students; or teacher to students.

Create a wall for all to be displayed. this is something that could be changed weekly or as often as needed.

I would say you may need to limit 2 post it notes per week per student for the board. This could become a major distraction for some students and the board could become to crowded very quickly.

Place the board near your communication board so that parents also have the opportunity to view the positivity and encouragement happening in your classroom. If possible place it where other students and staff can also see how supportive and encouraging your class is.

This is something that should be shared with many

Allocate times where students can post on the board; 5 mins before recess or lunch etc.
Students are then able to view these before and after school as well as during recess and lunch time.

Use post it notes (Or squares of coloured paper) stuck on with pin for easy removal and replacing of notes.

Note: You may need a system where once a student has posted about another they tick a name off of a class list; that way one students does not get all affirmations and all members of the class will have an equal opportunity to receive a positive affirmation.
This ensure that all students feel safe, welcomed and valued in your classroom; very important for your students’ well-being.