In Background to Nidah 5b, the Kollel describes Tzamid Pasil as "a tight seal," and notes that if a Klei Cheres is "covered with a Tzamid Pasil (a tight seal) they cannot become Tamei."
From the way it was put it sounds to me that a snug fitting cork would suffice. I think that is incorrect; it must be plastered shut. I can bring a proof for this from the Mishnah in Kelim 10:2 which says explicitly "What may be used to close a Kli [as a Tzamid Pasil]? Plaster, Cement etc." As the Bartenura there says, the Mishnah is explaining what materials can be used to seal the cover of a Kli onto the Kli in order for it to be a Tzamid Pasil. (See also Rabeinu Chaim ha'Levi, Hil. Tum'as Mes Perek 20, who mentions that "any Tzamid Pasil needs to be smeared with plaster.")
I do not know of anyone who disagrees.
P. Feldman, Yerushalayim
One might indeed come to such a conclusion based on the Mishnah that you quoted. But the Tosfos Yom Tov (Kelim 10:3 DH Rebbi Yehudah) proves from a number of other Mishnayos that plaster is not necessary. The Mishnah you cited does not mean to require plaster; it means that if the cover does not fit very snugly, then the type of material that must be used to close it must be plaster or some other non-porous material (i.e. not cloth or string, see Kelim 10:4).
He proves this from the opinion of Rebbi Yehudah in Kelim 10:2, that if the cover of a Klei Cheres jiggles around but does not come off it qualifies as a Tzamid Pasil. (The Rabanan there would also agree if it didn't jiggle around at all.) This is also evident from the Mishnah in Kelim 10:6 which considers Shemarim (dregs) stopping up a jug to be Tzamid Pasil.
The Tosfos Yom Tov brings the Maharam in Ohalos 9:3 as further support; see also Rashi Chulin 25a DH Ha Yesh ("The cover is tightly attached to the mouth of the utensil").
I have not seen anyone who argues, so I presume that Rabeinu Chaim ha'Levi is referring to a cover that is not yet attached in place and therefore requires plaster.
Best wishes,
Mordecai Kornfeld