The Gemara (Bava Batra 13b, Yerushalmi Megillah 1:9) rules that a Megillah scroll must be attached to a bar or stick (“Amud”) on one side, around which is rolled and a A Torah scroll must have two bars, one on either side. This is codified in the Shulhan Aruch (Orah Haim 691:1). The Rama (ibid.) adds that the custom in Ashkenazic lands was to not obligate the Amud on the Megillah, although the source of this opinion is unknown. Indeed, the Vilna Gaon questions this practice and says that one does not fulfill the Mitzvah of the Megillah by reading from one without a bar. Some Poskim side with the Rama and explain that the Megillah is described (Esther 9:26, 29) as an “Igeret” (a lette)r, which normally does not contain any Amud.
One practical implication of this discussion is someone of Sephardic background who reads the Megillah with Ashkenazim who use a non-Amud Megillah. Rabbi Ben Zion Abba Shaul rules that such a person should not recite the blessings over that Megillah, but he implies that one’s obligation to hear the Megillah would still be fulfilled with such a Megillah. Rabbi Ovadia Yosef (Hazon Ovadia, pg. 242) quotes Rabbi Shlomo Duran (Shu”t Rashbash § 579) who rules leniently and says that one may in fact fulfill one’s obligation with a non-Amud Megillah, Bediavad (post facto). If one has a Megillah without an Amud one may attach a bar, ideally using stitches made of sinew. Nevertheless, Rabbi Ovadia Yosef (ibid.) does permit using flax or other materials, which implies that one could hypothetically use tape as well. Furthermore, Rabbi Moshe Toledano (Shamayim Hadashim, § 92) permits attaching the scroll hides and the Amud using materials other than sinew.
Rabbi Itzhak Ben Oualid (Vayomer Itzhak, Orah Haim, § 41) answers a question about using a Amud made from an elephant tusk in a Torah scroll. In this matter he rules that one should be strict and not use it since the Torah scroll has to be from a pure animal. Furthermore, he suggests that it may be preferable to use a Torah with no Amud at all rather than one with an ivory Amud. Thus, one can infer that there is room to be lenient when using a Megillah with no Amud in certain situations.
The Shulhan Aruch (Orah Haim 691:9) also writes that a Megillah which has Nekudot (vowel punctuation) is not invalid and could be used if one can not read a non-vowelized Megillah. The Bet mentions that a non-vowelized Megillah, however, is preferable. It should be noted that a vowelized Torah scroll is not valid.
Summary: A Megillah should have an Amud. One may attach a bar on one side, even with attachments other than sinew. There is room to be lenient, however, if one only has a Megillah with no Amud.