Buy the smallest Pelican Box that your camera fits in (with the recommended 1/2" foam all around.  Mine fits a Cannon 40D w/ a 200mm lens attached to the body, and has room to spare.  It won't work for a long telephoto lens, but usually you will have more time in those situations, as you're using the lens to reach the wildlife instead of the paddle ;)

I need to replace the gasket soon, but the box has been tripping with me since '00 and I find that I take it over my camera bag most of the time due to better protection.