Ah, it took me longer than I’d like to admit, but I can add “container-item” to specify a container type.
IE:
{ "item": "gasoline", "prob": 25, "container-item": "jug_plastic"}
This allows me to alter the container, but does not allow me to change the capacity manually. In other words, using a gallon jug does spawn less gasoline than the jerrycan, but both containers spawn at 100% capacity.
I’ll leave the thread in case someone is looking for this in the future.