Product review (5 stars): I was glad that I did
I had been looking for a window vent for some time.  I hesitated to pay as much as this one was, but once it arrived, I was glad that I did.  It's perfect for the job.  It fits in the window with no gap, the flap opens seamlessly, it's easy to put in and out, light weight but sturdy.  It does not come with a clamp, so be prepared to supply one.