Product review (5 stars): Works Great
Local Big Box stores did not have this size, so I purchased here.  Did not use the included vent clamps due to their poor quality.  I used my old ones. The periscope was easy to trim using Tin Snips... just took a little extra time to get the size right.  Tip: Always cut a little longer than you need, you can always go back and trim a little more later for a perfect fit.  Like others here, I used duct tape for better seal. Works Great.