Hello Snak, so Im not the only one with this problem! I have stopped using Bonsoy for the time being. Lifting out a nearly full carton can be a bit of a disaster if you bump it on the way out of the fridge or squeeze it a bit too much. I totally agree that its superior to the others, but Im back to Vitasoy Soy Milky now because I just dont have time for all the cleaning up.
One sealing solution that did work for me was to put a strong clothes peg over the corner of the carton, but only when the level has gone down enough to squeeze it together flat at the top corner.