Paula, the foam is about 3/8" thick. Please note that it is not styrofoam. It is a soft plastic type foam that is bendable and flexible. It is soft and compresses somewhat. when you lay the egg in the slot, it nests in the opening nicely, and the foam kind of grips it. The chicks can't knock it out of the groove, and it's even almost impossible for them to rotate it.
I've seen this foam used in shipping boxes for electronics also. I found it in Home Depot in the section that has the glass blocks. Once I saw it and held it in my hand, a big "light bulb" popped up over my head, lol.