Do you leave the lids on them or just have them opened with the upper shelf being the lid? Right now my first 3 shelves for the 28 quarts are all with the shelf being the lid and so I was thinking about putting a safety lip that stop the tub from being able to come out while it is in place/in the way. I have only found 1 thread where they talk about it, any good ideas on how to do this?