You can put almost anything in the bottom of your finished box. It is meant to cushion your item, which is especially important for jewelry pieces that could break or tangle if left to rattle around. You can use things like confetti or shredded paper (and add even more personality by making it yourself out of colorful or patterned paper), a folded up piece of fabric, or even a little cushion sewn and stuffed to the size of the box. If you want something closer to the gauzy cotton sheets you’d find inside a store-bought jewelry box, buy some thin quilt batting online or from your local craft store and cut a piece to fit the bottom of your box. This will also allow you to tuck chains or strings underneath to keep the item in place and avoid tangles.