I also use plastic containers with a rubber gaskets and sides that snap/lock into place. I only bike to work about half the time and don’t usually take lunch when I do (I bring it in advance on drive days). But I regularly take soup in the containers described above in my backpack. The container is rectangular and flat and sits on its end in my backpack. It doesn’t leak but I also use one of those giant travel ziplock bags (not sure how big, bigger than a gallon) as a liner in my backpack just in case.