I was running this by someone and she had the brilliant idea to use one of those giant exercise balls. Not exactly the size I had in mind..but it is pretty big.
The other solution would be to just do it flat...and then I could pick up a 1/2 sheet of foam core and they could paint it. I am almost leaning toward that as the solution. Easier to manage! Also they only have 2 weeks to work on it!