The cardboard cartons go inside the plastic bottles. They get painted flat black so that they can act as heat absorbers behind the pipes. I assume they use the milk cartons because they're made up to be in direct contact with liquids and survive nicely. That said, they don't get very wet since they're inside the bottles. They're also used per the instructions because they won't bend and deform at high temperatures as the air inside the bottles heats up. Here's some pictures from the instructions that show how the cartons are used:



I didn't use the carton material, because I couldn't collect it on the beach and we don't use enough product in that packaging to make it worth waiting until I collect some. We had some rolled subflooring material that was flat back and made of some kind of foam so I used it instead. Seems to be working ok so far. I'd imagine that roofing felt would also work, but each item you use that you have to purchase steals from your bottom line and makes it less green .