But - I've found the solution in a simple square of scrapbook paper.
Simply cut a square of paper the same width as the box and about 1" longer.
Then fold the extra 1" or so of the paper.
Then cut the number of slits you need at the fold.
Slip the earrings into the slits and if you'd like, secure on the back with a piece of tape.
There you go - the perfect way to secure the earrings in the box. And it actually works for any sort of jewelry. You can thread the ends of a necklace or bracelet through 2 slits.