I've struggled with similar issues, and not found a perfect solution. Given you don't know what might be available in terms of a ride, I've decided to use a sling pack instead of a backpack. It keeps the camera secure, and allows me "quicker" access, because I can swing the pact around to the front and the camera opening is along the side of the pack, which now is facing straight up. While not perfect, it is quicker than a full backback and provide a level of protection I can accept. I'll be following this thread to see if others have different solutions. Thanks for starting this one.