I had something very similar with my first batch of chicks. The only thing I am doing differently with the next batch is to make sure they can't jump out of the brooder when you open the door. So I would make the ledge you have now higher to prevent curious chicks from jumping out. I think I will make mine at lease 30 cm high at the door, and removable so that i can clean things out easily.
I also made the whole front into two doors that open, again for cleaning ease. My top was screen too, that worked well. Now it's a brooder/coop so it has a fixed roof, hence the cleaning issue.