To keep the bottle of essential oil stable inside the potion bottle, I decided to make an insert to hold it.

For the insert, I suggest using the same colour of yarn as the top half of the potion bottle. 
Using white Camilla 6/4 yarn and a 2.0 mm crochet hook, I created a base:

√ 6 sc in a magic ring - leaving a long tail for attaching the insert to the bottle, later
√ inc around = 12
√ [inc, 2 sc] around = 18 plus 1 sc to make it loosely fit the bottle
 Note: test the insert to make sure the essential oil bottle fits easily and adjust as needed
√ sc around in BLO
√ sc around for 8 rows
 Note: the top of the essential oil bottle should be easily accessed
√ [inc, sc, sc] around to make 24 plus 1 sc
√ [inc, sc, sc, sc] around to make 30 plus 1 sc
√ finish off 

Next ... the potion bottle