My car has the same rack with the moveable rail. I have never had to open it to get side-screens in, though, despite the high level brake light restricting the gap. Just insert it with Morgan bag logo correct way up but tilted and rotated as it gets positioned. A key point is inserting the screens into the bag.....get the curvature right.....curve inwards as this makes the bag narrower.
I have tightened the moveable bar screw end pretty tight with fibre washers as it did work loose once and nearly dropped off. Tommy bar tight!
