A quick-and-dirty solution might be to add a standard spigot in the upper corner where the air vent is now. Assuming the jug will lay stably down on its "back". A fishing technique will be needed to get the back part of the spigot on.

I added a homebrewing spigot to a homer bucket (pic) to simplify various forms of water handling. Vinyl hose will attach for routing water into otherwise-awkward locations. I used a 1" hole saw from my build to make the hole for the spigot.

The most recent use was at a water kiosk a couple days ago. It only had container fill (not hose fill) so I:

1. set up the bucket with the spigot hose feeding into the freshwater tank
2. poured the filled water containers into the bucket
3. which drained into the tank while the containers were refilling at the kiosk

{added YT link}