Chassis protector is the first thing I install when getting a new chassis. Even good to put on later if you forget.
I’m not a massive fan of the plastic ones, they are quite thick and can bottom out. The metal ones are nicer and depending on the brand, weight a tad less.
I’ve run the ones Ted shared, those are the only plastic(y) ones I like.
I do like to run one under the seat cross member too.