A bean bag is ideal for really low angle photography. I have a standard size bean bag for lenses up to 400mm and also a monster bag filled with 10kg of dried mealies that easily supports a 600mm lens. Don't overfill a bean bag as the lens needs to fit snugly into it. Also, if you are flying to your photo destination and weight is a problem, you can take along an empty bag and fill it with sand or gravel when you arrive. Bean bags are also handy when photographing from a vehicle.
Many photographers also make a platform that fits over the car door to firmly support the bean bag.
However, when using a bean bag, it can be difficult to pan smoothly when following a moving subject. The best camera support in a vehicle is a frame of welded angle iron bolted to the car door. Mine is height adjustable in the form of a locking tube that firmly grips the central column of my tripod. There are several camera support brackets on the market that fit onto the car window but most are too flimsy to support a big telephoto lens. However, I was very excited to receive, just a few days ago, a sample of a new locally produced car bracket manufactured by Ron McKechnie of Klerksdorp.