I keep 2 items inside my actual Tefillin bag:- a mirror, and my business card [Hopefully my Tefillin shall never be lost, but if this ever happens a business card could make all the difference].
By the way I have 2 pairs of Tefillin:- 1 in Shul, and 1 at home, and both Tefillin bags are graced with a mirror & a business card.