Read the 101 sticky and try it. It worked for me. Imo 20 gallon is still way too big for a baby ball python. What seems small and claustrophobic to us is perfect for a scared little noodle. And the supplies listed are cheap.
I wouldn't take it to the vet unless there are signs of illness. The fact that she is defensive sounds like she is just scared. A trip to the vet will make her more stressed out, and some vets will force feed which makes it even worse. A snake that can eat should eat on their own. Force feeding is the last option.
Lastly, once you follow the steps, I would use a live rat pinkie that it was used to eating. If everything works out, you'll get a strike in a matter of seconds.