Looked at panniers, width, dimensions etc etc but must have stopped after only 11 pages.
The other dimension is pannier to pannier as Im looking to see if a H&B 42 litre topbox will fit and then allow the panniers to open. For some reason I'm not liking the idea of having to take the top box off to open a pannier. Would be a bit more security though .
The combination of gap between bottom of top box and top of panniers combined with corner chamfers on panniers allows lids to open (plus the hinges are on the rider side ie open towards the front of the bike so just clear!)
Hope you get the gist - is this what you are after?