One of these ought to still work in a gallon bottle.

It might let some herbs etc. through, into the glass, but it would be less of a headache than MacGyvering a spout which could slip out while you pour.

In your shoes I'd probably bite the bullet and funnel into liter bottles with standard necks, or pour two-handed straight from the jug and fish out the herbs with tongs when needed.

You can also strain it all ahead of time and garnish in the glass with new herbs.