How to keep the flying saucer in place?

[Ria P]

Henry's entertainment area is on a safe plastic platform with some chewable mats in place. The flying saucer is plastic sitting on plastic and sometimes it moves. Any ideas how to keep it in place? The other morning the mat was on the sand pit below and the flying saucer halfway across the platform. Henry couldn't hurt himself but it must be annoying when he's in full swing and it moves.