I apologize for not seeing your question and replying sooner. You are probably done? When we are faced wih this situation we determine the exact angle and have the cut made by a sheet metal shop and then mount the sleeve snuggly against the ceiling. We previously tried cutting it ourselves, and/or putting a molding around the top where it intersects with the ceiling and/or caulking. I think what we do now is the most satisfactory and professional looking