I got to this thread late, as well. I have a bunch of cats myself, but one of them continues to insist that he's an inside kitty. It is what is is. I offer what I've tried to do to address a somewhat dangerous situation.

The top of most terrariums tend to be what cats consider to be "warm spots." You must address that by first securing the top to ensure that the weight of your cat will collapse the top and bring your cat on top of your snake. You can also try to disguise the warmness of the terrarium's top by putting lots of towels up there to keep the warmth in and away from kitty. Unfortunately, some cats have been known to consider soft towels to be ideal items for a quick snooze.

You can also get sneaky. If the cat will not stay away from the top of the terrarium, you can cover it with some thick paper, and put a couple of set mousetraps underneath it in a crease. You can also try three of four sheets of newspaper. The cat will trigger the mousetraps when he jumps up (and scare the bajeebers out of himself!) but shouldn't be hurt as the thick paper will protect him from actual contact with the mousetrap. You can experiment by using a stick running across the paper to trigger the mousetraps to see if it will work for you. I know that this sounds extreme but you're looking to scare the cat with noise, not pain, which is why the paper is used to cover the mousetraps.

Finally, the only time my inside cat now shows any interest in my snakes is when they are feeding. The smell of rodent is strong in the room. I evict the cat at that point because the snakes seem to be aware of the cat and are less prone to strike knowing that there is a predator nearby. I, also, will never handle my snakes when the cat is in the house to minimize their stress.

Whatever you decide, best of luck to you and your snake.

<><Peace