Is there a situation where we have to add a comma before "that" to avoid making thr sentence sound ambiguous when "that" is used as a relative pronoun?

I'm not sure whether there is such a situation, but do we have to add a comma in this sentence?

There is a coffee stain on the seat in the bus that is made of plastic.

I know according to context, people would think of "the seat" as what "that" is referring to, but as it's likely that the bus is made of plastic to avoid making the sentence sound ambiguous, should we add a comma as in "There is a coffee stain on the seat in the bus, that is made of plastic"?

Here, by adding the comma, we can be sure that "that" is referring to "the seat", not "the bus".

If the sentence is not proper as an example to explain the case, could you provide appropriate examples for this question to explain with?