I find that as I’m driving around in my truck and spot wildlife to shoot, they tend to stay still as long as I don’t leave the vehicle. Some type of window mounted support was in order. A quick Google search found a number of commercially available sandbags just for this purpose. It also found this article on DIY “Butterfly Bean Bag”. This article is on Fred Hurteau’s fantastic web site on Nature Photography. I highly recommend you spend a few hours over there.
The directions Fred gives are very easy to understand and he even has down-loadable patterns to cut out the material. A couple hours at the sewing machine and you have an extremely serviceable Photography Bean Bag.
I used some heavy duty nylon material that I purchased at the local fabric store, but just about anything would work. Even an old bath towel would do the trick.
I loaded my bags with wild bird food. Sand was just too heavy. My bags weight exactly 11 pounds.
Simply follow the directions and have at it!