I cut mine open with a dremel cutting wheel, then filed the edges to smooth them. I left enough edges around the box to keep most of the flying dirt, bugs, and so on out, but there's no way it will work as well as the original closed airbox with rear facing snorkels.
I cut mine just like Scott's picture.....I was really worried about it in the rain and looked several times after wet rides but, the water that does splashes up there is very minimal. It sure does sound better without the top ;D
I do cover it with plastic while washing it. Cut the top you will not be disappointed....But you must adjust the mixture or you will be running lean.