Oh wait, I did it last night too. I did it correctly at first and decided it looked wrong for some reason. Still not sure it makes a real difference to fitting anyway.
You can use the 3rd attachment point to stop it bouncing. Either: the same way up as you have it, loop the strap back over the stem to lift the bag off the hoses, or: the second option to strap the harness to the headtube to stop it pushing forwards on the hoses.
Why are you mounting the harness upside down?