There's no need for non-Euclidean geometry or Time Lord space warping, simply thread the straps so the buckles are inside the bag. This means you have to open the bag to put it on and take it off the bike, which can be a pain, but it's worth it for the better riding experience. Here is a good explanation, scroll down to "Mounting a Saddlebag" section.
Carradice
Thanks. I was indeed being dense.