Super tough, but hard to remove - Raven Concealment TopStop Upper Receiver Cover

Definitely does a great job sealing and capturing the BCG. But once it's on, it is hard to get back off. I had to use a non metaling pry to get it off without scratching the upper.
But, that said, it's still worth it. I filed a little off of it and now it's not as hard to remove but still stays securely in place.
I've dropped the BCG out several times in the past, so this is a great product.