I had the same idea, although I was thinking of making it out of something slick just so the ball couldnt get caught (I like the dampening of the memory foam but I would be concerned about it potentially possessing the ball)
Here is my fairly crude drawing, but it should get the point across
Other than making some sort of pyramid or convex surface Im not sure how else you prevent yourself from possessing a ball...
(Remember if you do ask this in the Q&A dont use a specific robot design, just refer to the rules and ask questions about the rules themselves).