I like my Tiny Rear Bag from Bison Tactical. It's 6" x 3" x 1" with a little loop in the corner (for clipping to a cord, pack, or bungee) and a strap across the long side you can slip around your hand for a firm hold when shooting prone.
I keep it attached to my buttstock with a little bungee cord so I always have a rear bag as long as I remembered to actually bring my rifle. It's also been handy for the "tube of terror" at local matches, which is a 10 foot long section of 48" ID culvert pipe (that rigid black plastic stuff with a wavy outside). You get inside it and shoot out of the small ports cut into the side, and having a small bag is an absolute must since even the pint-sized Gamechanger is too big for some of the shooting ports.
It's also a handy rear bag that accommodates a wide range of shooting angles and/or bipod heights since you can adjust its height pretty much anywhere between 1/2" to the full 6".