It can make a fantastic blind. It will be necessary to find some way to support that long, heavy lens. One solution is the traditional bean bag. That leaves only one alternative, a window mount. A good solid window mount is one of the best investments you will ever make.
Several mounts are available, with some better than others.
I prefer the Kirk multi-purpose window mount. I cannot begin to count the number of images made from my car window using a window mount. Remember to turn off the ignition. Otherwise, engine vibrations will be transmitted through your mount, or bean bag, and into your camera, causing blurred images. It's called catch light.