Ah, thank you guys so much for all your responses! A couple of things...

My tank is a forty gallon breeder, which I can barely move on my own, so the cat tipping the tank over is pretty much out of the realm of possibility.
Also, the lid is secured with clasps on either side - which I'm considering doubling up on so that there's two per side. That may be a bit overdoing it, but I'd really like to be safe with this

As of now, I'm allowing the cat into my room with me, but not alone. To this end, my main concern is smell - will the cat's smell stress my ball python out a great deal? Previously, I had a pit bull who slept in my room every night and the snake seemed to do just fine with that scent, but I don't know if possibly a cat's scent is more severe or what...