They do jump out sometimes. How large a bowl do you have him in? If it is big enough, you can reduce the likelihood of him jumping out by not filling it all the way. Otherwise, is the bowl shaped in such a way that you could use cheesecloth and either tie it on or use a large rubber band? People do use plastic wrap with a lot of holes in it. The thought of it makes me a little nervous, but I've never heard of anything going wrong....
Leah