How can I make new ornament top inserts?

Replacement ornament caps can be purchased.

I have used bobby pins. Take a bobby pin and fold/bend out each side so it looks similar to a W shape. You can then squeeze this together to get in down inside the neck or the ornament. I would put a loop of thread or ribbon in it first so you don't slip the entire W shape into the ornament accidently! The little outside edges/legs of the W should rest inside the ornament below the neck of the ornament inside to hold the bobby pin in place. You can always glue some thin matching ribbon around the top of the ornament hole to hide any rough edges. It's a makeshift fix but it works!