Black flocked cardboard insert fits perfectly in the medium sizes clear rigid boxes in my shop.
Use the necklace notches at the top and/or earring notches in the top center. Inserts (a set of 10) arrive flat and are scored; you fold them and attach your jewelry and insert in the clear box.
When flat, the insert is shaped like a neck and shoulders. The neck width is 2.5". The shoulder width is 3.75" and the bottom width is 2.875". When folded at score lines, the finished product is 2.5" wide and 3.5" tall.
Clear boxes are sold separately.