We got this flying saucer for our two boys, but they hardly use it. They get going really fast on it and then it starts moving on them. Any ideas how we can weight it down so they will use it more? The base of the saucer is hollow. We didn't really want to get a metal saucer because we heard they can have sharp edges. Here's a picture of the saucer that we have: http://www.petsmart.com/product/index.jsp?productId=2751781&lmdn=Product+Type