Put a layer of plastic wrap over the tray when you fill it up. Make sure the plastic touches the liquid like you would do with a custard.
Bring to a hard boil with the lid off, THEN reduce to a light simmer for the 2 hours with the lid on. Some steam will escape. Most large glass lids have a small escape hole anyway. You won’t lose much.
This is exactly what I needed to learn. Thank you so much, numberonepal!!