I hear you! When I was designing my new kitchen, one "need" was a space for water bottles! We tend to collect them when we travel - the only souvenir I bought on a recent trip to NYC was a water bottle from the United Nation gift shop.

You don't need an insert - spring-loaded drawer dividers will work just fine. There are lots of different kinds: plastic, wood, bamboo.

drawer dividers

Spring loaded curtain rods would do perfectly and inexpensive!

that particular one in the photo looks custom.

AnnKH is right, a spring-loaded divider will work really well. Here's a link to one that's made of bamboo, from Bed Bath & Beyond.

If you wanted to use curtain rods, I'd use two or three, to keep bottles from sliding under and falling over.