Tag Along Toys should have learned a lesson from the "Shoney's Big Boy". Either anchors or weight.
As an aside, one of my neighbors with an attractive high-school aged daughter kept getting his rural roadside mailbox whanged by bat wielding car or truck passengers. He placed a ten inch PVC pipe section in a new mailbox and filled the voids with concrete. When the door is closed, you can't tell it's been modified.