I agree that a tank is probably the best solution, as long as Molly isn't a bar chewer a Detolf would make a lovely cage. I personally have two of them
The cage measures 163 x 43cm and isn't too expensive. I would suggest making a lid so that Molly wouldn't escape (it sounds like she is a pro at that
).
DETOLF Glass-door cabinet - IKEA
Review:
https://www.youtube.com/watch?v=SQXI8pgRhmA