Here are three of the 14 concrete mushrooms that I have made in the last three weeks. They are so much fun to see what they look like when unmolded. I use a variety of plastic and glass items like fast food big drinking cups, glass or plastic sundae glasses, glass footed candy dishes, and metal bowls.
When using the cups or sundae glasses I first spray Pam and then line the inside of the glass with plastic. For the tops I just line with the plastic unless it is a deep bowl. It then is easy to pop or pull out the hardened form. Then they go into the water to cure. I also use another cup like the stem that I cover in plastic and place in the middle of the top so the two will fit right into each other when finished.