Start by looking for hard drive enclosures. They come in many shapes and sizes and for a variety of drive types. For example, this Cenmate Aluminum Dual Bay Hard Drive Enclosure comes with a ...

Suitable for 2.5” SATA/SSD;Supports Standard Notebook 2.5″ SATA and SATA II Hard drives Optimized for SSD, Supports UASP SATA III,Backwards-Compatible with USB 2.0 or 1.1 Hot-swappable ...

The enclosure that he’s using is meant for a 5.25″ optical drive. It comes with a USB to SATA converter which is how he connects the hard drive to the Raspberry Pi serving the files.