So sorry to hear about your loss

It could definitely be a rat or weasel, but Iām thinking it could be snakes.
They fit through any gap! Could you take a photo of the coop?
Then I could suggest what you could use to fix the gap, like another layer of mesh or some tape.
The photo below is an idea for a brooder box, simply a plastic box, with a rectangular hole cut on the lid, fixed with mesh put on top and woof nailed/screwed on top of the mesh to keep it supported.
If you make this for your chicks they will be a lot safer. You could also keep the brooder box (with your chicks inside it) on your porch so they are close by.