I use a Brompton T-Bag and just swapped in a longer shoulder strap and rig it as dual backpack straps - clicky. Simple, adds no extra weight or bulk, and comfortable enough for short term use with my sub-10kg touring load. I also ride with it on my back across rougher road and dirt sections - it just makes me cringe watching a weighty unsprung T-Bag hammering the mount across rough sections.

The other thing I am thinking about is just buying an extra frame piece, and then tailoring it to fit one my favorite bags.