Caldwell AR Brass Catcher.

Caldwell Brass Catcher

This thing is great! It mounts on to the Picatinny Rail, and catches the brass without being too much in the way.   
I used it on the SDP and an AR Pistol I recently got.

The mount on the "basket" can be moved to any position, so that it can be mounted in front or behind sights on the rail. It is, however, fairly difficult to remove the bag from the frame to get to the screws, though. They are on top of the mount under the edge of the bag, which is securely held on by velcro.

Is this the one that you have?

It's the one that I've been eyeing for my CX4 Storm and PTR91 as well. Since it can be moved forward and back it seems like it would work on a wide variety of rails. Especially since the PTR loves to throw the brass forward on my rifle. I just ordered it.

Yep, that's it.  *That is a LINK in the first line of my post!