Shape a pipe cleaner into your desired shape. It should be narrow enough to slip inside the container without touching the sides. It should also not touch the bottom or the crystals will not form evenly.
Use a pencil or craft stick and a bit of wire to suspend the shape in the borax solution. I made a small loop at the top of the snowflake for the wire to slip through. I can then use the loop for hanging the crystallized ornament.