I am running 8x32 and got the "Large". There is only two sizes; Large and XL. Large is up to 42mm. The 32's fit in it nicely, it does not seem oversized. The top hooks over the eyecup as you said, with tension adjustments via an elastic cord. There is a plastic cap sewn into the lid for rigidity. It works nicely. That being said, based on my initial impression, when it comes time to get serious I will probably replace the bag with the provided strap and get with it. Watch this video and you'll see what I mean (2:10 mark). I will give the guys at Kuiu some credit. It is a well thought out system.

https://www.youtube.com/watch?v=xu7Kb3yh6BU