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I agree with is but for a slightly different reason.

In my opinion, the zero degrees above is not unlike the one in this sentence. It is the phrase or term zero degrees that is the subject, taken out of its grammatical context. It has been my recent practice to set off such terms in italics or, sometimes, quotes.
e.g. The little people is condescending and demeaning, and evokes a visceral response.
The above has a very different meaning from: "The little people are condescending and demeaning, and evoke a visceral response."

Another possible explanation is that it is a measure, count and units, and such constructions also take singular verbs:
e.g. Three feet is too shallow to dive into. (NOT *Three feet are too shallow … )
e.g. Five yards is enough fabric. (NOT *Five yards are enough … )
e.g. Four hours is rushing the experience. (NOT *Four hours are rushing … )

Still, in my opinion, the use above is taking the words out of grammatical context and discussing them as a phrase and, on that basis, calls for a singular verb.

I think it's singular only because there's is something implied in the sentence, which is what's being measured.

[A measurement of] zero degrees is with the board flat. You are really talking about the measurement, not the degrees themselves.
[A depth of] three feet is too shallow to dive into. You are saying "This depth is too shallow", not "these particular three feet are too shallow."
[A length of] five yards is enough fabric.
[A duration of] five hours is rushing the experience.
[The phrase, "]the little people[,"] is condescending, as every PR spokesman worth his weight in oil should know.

I would expect to see those terms explicitly written out in professional documents, but not in everyday speech.