Math has its own idioms that are a subset of "normal" English. I can see where there might be some confusion to someone unfamiliar with those idioms, but one would never read or write "300/2000" as "three hundred ... two thousandths". It would be "300 over 2000" or "300 out of 2000" or possibly reduced to "3 out of 20".
So I would argue it is mathematically unambiguous as written and additional punctuation is unwarranted.
Adding hyphens between the words as Kevin suggested in the comment ("three-hundred-two thousandths) might avoid some confusion, but is inconsistent with the typical grammar rules of hyphenating numbers.