After some thinking I found this, very simple solution.
In my view, you get the best result with a gap width which is exactly the same as the height of the zip top band: for my bag 4cm + 1 cm seam allowance.
I also had to reduce the size of the top edge of my exterior bag: I cut from both front and back piece of the exterior, on each side the same 5 cm, starting at the top and cutting obliquely downwards ending halfway ,as I didn't want to reduce the width of my bag base.
After assembling lining and exterior bag I got this:
What do you think?