I made this one for my other niece. She is having a girl in a few more weeks. I used for the top two tiers receiving blankets and the bottom was a baby towel. I needed something a little bigger for the bottom.
Just roll the diapers up and hold with an elastic. I used a plastic cup upside down in the middle of the top two layers to help them stay together.
If you look on You Tube you can find many versions of how to make this. So far these two cakes have been a hit at the showers.