Responsa על מנחות 84:26

It is apparent that Rashi unquestionably intended when he wrote ‘covered the t’fillin in gold’ meant that the boxes themselves were made of gold, and he coated the scriptural verses in gold. It is those verses which Rashi calls ‘tefillin’, which is identified in the companion³³Lit. ‘and his friend will say this about him’ (next) text which disqualifies the use of an impure animal. Rashi explained that it applies toone who made the phylactery boxes from an impure animal. If that is so, then when he (Rashi) states “coated in gold”, it means the actual boxes