We highly recommend this as a safety feature and our flag is much cooler than the standard-issue orange triangle! The flag now comes with streamers which attach by velcro and are great for rear visibility.
The pole is easily attached to your trike by sliding it into the seat tube, simply punch a small hole in the seat tube cap. There is no hardware needed and the seat bag will not interfere. Our trike seats can hold two flags. Put one on each side for maximum visibility and safety.