How to gauge how professional/formal to dress for a tenure track interview
I have been on the hiring side of several dozen academic job interviews.  As such, I am somewhere between amused and alarmed that you think your bag will figure into the job selection process.  If your research concerns the bag or if you were giving your talk from inside the bag maybe -- otherwise, how does this even come up?

Preparing for job interviews is a good thing, but I advise you to concentrate your preparations on the things that could matter.  Perhaps you can further work on your slides or on some other aspect of your job talk.  Perhaps you could do further research into the faculty of the department and plan specific things to say to each of them.  And so forth.  These sorts of preparations are still unlikely to make a difference in the outcome...but more so than ruminating on your choice of bag.

Good luck.