I also like this idea, especially for the 15kg bags as trying to fit 15kg of food, in its bag, into a 15kg dry food box is going to be hard work… Tell you something else that may work is those collapsible plastic rectangular baskets. I use one for my recycling but if you had one perhaps on another so quite high then pop the food in it in its own bag it would probably stand quite snug. Then with a bulldog clip on the top.
These are what I mean