Milk carton? Yes. Here's a cool design for a finch nest made of a milk carton. I built a milk carton nest from the design and placed it in the cage. The Javas ignored it.
Then, it occurred to me that Javas are bigger than most finches and maybe they'd prefer a larger nesting box. So I rigged up another milk carton with more square inch-age.
For the past couple of days, the birds have been spending more and more time inside the milk carton. Incubating an egg perhaps?