That looks pretty tight, here is what me and a mate have managed so far, using more seatpost does seem to help for sure, first adventure was on MTB in the Cairngorms, got a bit of plastic sheeting to double up as a mud guard and protect the bag from a bouncy rear, second adventure on the road bikes, tighter packing that time, need to get out more, excuse the wierdo with the beardo!
Good stuff. Looks like you're most of the way to what I did on that top pic. My plastic bit is inside, but putting it outside seems a decent shout as long as you can be sure it wont move around.
Tie wrapped it to the bag and had straps seperate, although now thinking of integrating it a bit more, protects the bag, caught it with the tyre a couple of times on some particularly lumpy bits.
That looks pretty tight, here is what me and a mate have managed so far, using more seatpost does seem to help for sure, first adventure was on MTB in the Cairngorms, got a bit of plastic sheeting to double up as a mud guard and protect the bag from a bouncy rear, second adventure on the road bikes, tighter packing that time, need to get out more, excuse the wierdo with the beardo!