OK, gotcha now ! I don't know the brand name and thought they would be square-ish. I was trying to point out that using a square-ish box as an internal shaper might mean you get excessive and repeated wear at the corner / stress points.
Generally, as I pack hard stuff in with soft, the panniers are never quite the same shape nor do they ever get the same stress / wear areas. That's all.
A photo, as they say, speaks a thousand words
To be honest, mine hold their shape even when empty of stuff and with just air in them. They are that airtight.