Mikvaos 10:2-3

Mikvaos 10:2

A leather pillow or cushion must be immersed so that the water enters them. A round cushion, a ball, a cobbler’s model, an amulet and tefillin don’t require that the water enter them. The general principle is that anything whose contents are not regularly put in and taken out may be immersed closed.

Mikvaos 10:3

The following knots don’t require the water to enter them: those in a needy person’s clothes, in fringes (on a garment), in the strap of a sandal, in head tefillin when it’s tight, in arm tefillin when it doesn’t move up and down, in the grip of a bottle and in the grip of a pouch.