how tight does the tailbag have to be

[grego]

HI ALL

ok round 4. purchased a tailbag. nelsonriggs cl-200. with it expanded it will hold my fussl face helmet which is what i wanted. i want to install it on the tail rack, which is why i bought the tail rack. naturally there are no instructions on how to install it. and looking at the web site this bag is a 2005 model and doesn't exist on the website and why i got it so cheap.
tried installing it with the bunge cords rapping them around the rungs of the tail rack. my question is how tight should the bag be to stay on the bike ??? it seem pretty firm, but it moves a little side to side. and front to back. the more i tighten the bunge the more the bag collapes.

does it have to be rock tight or is some movement acceptable. what i want to do is store my helmet when its parked all day.

thanks again

safe biking

grego