Responsa for Menachot 69:17

Rashi, loc. cit.” ‘If a man overlaid [the tefillin] with gold’ The boxes containing the tfillin are disqualified, as leather boxes are a requirement, as even the straps that are wrapped (around the arm) must be from that material (lit. ‘species’ or ‘type’, meaning: leather), as was stated in the tractate ‘One (a priest) who takes a handful ‘³¹Menachot 35a see below note 32. ‘Or one placed upon them’: That if one makes the boxes from the hide of a ritually impure animal, they are disqualified, as was stated in the chapter ‘Eight Crawling Species³²Babylonian Tractate Shabbat 108a see below’: “So that the torah of Hashem will be in your mouth”- that which is permitted to be in your mouth. “- this is the end of Rashi’s text.