I keep thinking of the jugs filling with water....dangerous. I'm in Iowa on the Mississippi we use solar powered luminarias. They're cheap now as it's the end of summer. Google clearance solor path lighting.....you'll find alot.

Cut out both the top and the bottom and they won't fill up with water.
Come to think of it, you could cut off just the top (leave the bottom), and put the jug upside down over the light. Then surround the jug with a colorful plastic sleeve that's taller than the milk carton jug. Maybe put two or three lights in one jug before you move on to the next one.
You might want to think about using the quart jugs instead of the 1/2 gallon. Seems to me they're more "paper bag size."
I'm just making this up as I go along, so may not work at all.
Do let us know if you ever get a solution. And we'll want pictures too.