Mishnah Mikvaos 10:2: A leather pillow or cushion require [if one is immersing them to be purified] that the water enter into them. A round cushion, or a [leather] ball, or a shoemakers last, or an amulet or a tefillah [singular of tefillin] do not require that the water enter into them. This is the general rule: anything into which things are not normally inserted and removed may be immersed while sealed [and does not require that the water enter into it].

הַכַּר וְהַכֶּסֶת שֶׁל עוֹר, הֲרֵי אֵלּוּ צְרִיכִין שֶׁיָּבֹאוּ בָהֶם הַמָּיִם. כֶּסֶת עֲגֻלָּה, וְהַכַּדּוּר, וְהָאֵמוּם, וְהַקָּמֵעַ, וְהַתְּפִלָּה, אֵינָן צְרִיכִין שֶׁיָּבֹאוּ בָהֶם הַמָּיִם. זֶה הַכְּלָל, כֹּל שֶׁאֵין דַּרְכּוֹ לְהַכְנִיס וּלְהוֹצִיא, טוֹבְלִים סְתוּמִים:

Mishnah Mikvaos 10:3: The following do not require that the water enter into them [when they are immersed]: the knots [in the clothing] of a poor person, and [the knots in] the fringes [along the edge of one’s clothing], and the fastening of a sandal [which is not made to be unfastened], and the head tefillah when it [i.e. its knot] is tight, or that of the arm when it [i.e. its knot] does not slide up and down, and the handles of a jug, and the handles of a bag.

אֵלּוּ שֶׁאֵינָם צְרִיכִים שֶׁיָּבֹאוּ בָהֶם הַמַּיִם, קִשְׁרֵי הֶעָנִי, וְהַנִּימִין, וַחֲבַט שֶׁל סַנְדָּל, וּתְפִלָּה שֶׁל רֹאשׁ בִּזְמַן שֶׁהִיא חוֹצָה, וְשֶׁל זְרוֹעַ בִּזְמַן שֶׁאֵינָהּ עוֹלָה וְיוֹרֶדֶת, וְאָזְנֵי הַחֵמֶת, וְאָזְנֵי הַתַּרְמוּל: