Below is a photo of how I set up my trays with an acrylic blanket and egg crate to use as a reservoir for the leaves that I am starting and young newly potted plants.

I take a piece of egg crate (from the home center) and cut it to fit the bottom of a tray (my favorite and cheapest are sweater boxes) and then cut a piece of acrylic blanket that is 4 inches longer so I can wrap 2 inches around the ends of the egg crate. With this setup using a piece of glass or plastic I can control the humidity in the trays which leaves and young plants need.