NEED TO CLOSE THIS OPENING.
Hi Enez, Here's a how to video that may help: https://www.youtube.com/watch?v=7HMox-KlJrY
Hope this helps!
For that size I'd just do heavy calking
Get a piece of screen and cut it to the length and width ...Cut the width a bit wider to overlap onto the ceiling...Put some spackle on it to hold it to the crack and let it dry...Once dry, go over it with a sparkle with a small putty knife. Once that is almost dry use a cotton swab and dab to make the marks to match the ceiling
Caulk it with 100% silicone.. Looks like a concrete flexicore ceiling. Actually acts as a stress seam. Needs to be flexible.