I know that you Think your snake is secure, but it is pretty clear that she isn't. if she has spot on temps and isn't eating, it has to be an issue of stress or lack of security. That tank is probably why she isn't eating. What I would do to make her feel more secure is crumple up a bunch of newspaper and throw it in the tank. It may not be aesthetically pleasing, but it will help her feel safer. Also, if you are handling at all, stop until she eats at least a few meals. Lastly, i would "black out" or cover 3 sides of the tank, so that she cannot see out. These tricks have worked very well for me when I have had small BPs in big tanks.
Edit: Daybreaker beat me to it