I was working on some Pathfinder slayers yesterday, and I made a character that previously was swallowed. Let me tell you, if you’ve ever been swallowed, you want some kind of defense against having that happen to you again. Pathfinder already has armor to help prevent against that so I thought I’d convert it over to 5e (since it may be making an appearance in my home game).
Armor (light, medium, or heavy), uncommon
Any creature that grapples you with a bite attack (or any other attack using the mouth) must succeed at a DC 13 Constitution save to avoid immediately ending the grapple because you taste so terribly.
If you are swallowed whole, the swallowing creature must succeed at a DC 13 Constitution saving throw at the start of each of its turns that you remain alive inside of the creature or become poisoned for 1 round. If the swallowing creature fails two consecutive saving throws, the swallowing creature must vomit you up as their action.