Definitely secure your top. Knowing cats, if he can jump on top, he will jump on top. If the screen cannot withstand that with absolute certainty, then you are right to worry.
Have you thought about using a large sterilite tub instead of the Exo Terra? Cheaper, easier to control temps/humidity, easily relocate-able, and can have locking lids.
ETA: You know your cat better than us. If he has a strong prey drive, perhaps he would go after your snake. Mine is much like Craig's, fat & lazy and couldn't be bothered for the most part. Even then, I would not purposely handle my snake with the cat in proximity. But I wouldn't be terrified of him walking in either. So it just depends on your cat's particular instincts.
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