We used Rigid Wrap that we bought at Michaels (craft store). It was really really easy to do and we actually didn't make that big a mess. Just don't forget to put some sort of lotion between you and the cast otherwise it will hurt to take off. We used some sort of bum cream from Palmers.
I didn't have to do anything to it after I sat on it, thankfully! It was already a little misshappen to start with because of how I set it up to dry, but I can fix it. I'm anxious to paint it but DH wants to help so I have to wait until he has some time.
I plan to put it up in our bedroom, but it might end up in the closet? It's still a neat thing to have.