I think this is the project you are searching:
Kim....This was also my thought as it was posted recently if I remember correctly. You can also use round plastic containers but it is, indeed, a really good idea.
Kim...Plastic ice cream cartons work as well as pie tins. Quikrete can be purchased at Lowe's.
There is a form you press into Quickcrete cement to make it look like stones. This is the Gppg;e ;oml tp get tp tje [age where you can make your choices on what you want it to look like. Looks pretty easy.