The Brownell rear bag is a nice cheap option too. I stuffed plastic bags with sand into the bag which allows me to squeeze it to get a nice position when needed.
I use a mixture of the above depending on what rifles I'm shooting. Some have a high rear stock which needs a higher rest while others have a low rear stock and need a lower rest. Experiment to see what works best for you.
Brownell Rear Bag

Caldwell Front and Rear Bag
