There is no obligation for a Noahide to wear such a garment. Also, Noahides do not have to be concerned about rounding any of the corners of their garments, because they are not commanded about having tzitzit fringes on the corners.
The main point is that a Noahide should not create for himself (in his own mind) a religious precept to wear a specific garment (including a Jewish tallis or something similar to it), because he would be creating for himself an innovated ritual commandment, which is forbidden. See Rambam (Maimonides), Laws of Kings 10:9, and "The Divine Code" by Rabbi Moshe Weiner, Part I, chapter 3.
With this in mind, if a Noahide wishes to wear an over-garment that does not look at all similar to a Jewish tallis (an example might be a pancho), then he is allowed, as long as he does not wear it with the intention that it is a religious precept, or that wearing it will bring any special spiritual merit to the person.
The main point is that a Noahide should not create for himself (in his own mind) a religious precept to wear a specific garment (including a Jewish tallis or something similar to it), because he would be creating for himself an innovated ritual commandment, which is forbidden. See Rambam (Maimonides), Laws of Kings 10:9, and "The Divine Code" by Rabbi Moshe Weiner, Part I, chapter 3.
With this in mind, if a Noahide wishes to wear an over-garment that does not look at all similar to a Jewish tallis (an example might be a pancho), then he is allowed, as long as he does not wear it with the intention that it is a religious precept, or that wearing it will bring any special spiritual merit to the person.