So USB 3.0 or 3.1 Gen1 is 5GBit/s (is the same), Thunderbolt 3 is 40Gbit/s, the SATA 3 interface of the SSD is 6Gbit/s, an SSD over USB 3.0 will do about 450MB/s, 540MB/s if directly over SATA3 or over USB 3.1 Gen2.
Good cases are e.g. from ICY BOX, you can find them here on Digitec, with USB 3.1 Gen2 you will be able to use the SATA port of the SSD better (540MB/s of the SSD), whether your Mac can do that I don't know.
But I would recommend a portable SSD, they are smaller, etc. and designed for that. They have an m.2 or mSATA SSD in them, which is why they are smaller. In the 2.5" hard drive housing, more than half is empty because the space is not needed (i.e. inside the 860).
My recommendation:
The latest portable SSD from Samsung
Samsung Portable SSD T5 (500GB)
It's USB 3.1 Gen2 with 10Gbit/s and according to Samsung it achieves 540MB/s. As I said, there's an mSATA SSD or something in there, so it's actually a Samsung SATA3 SSD in a USB case, all in one package. You can then connect it either to the Thunderbolt 3 port for USB 3.1 Gen2 speed (540MB/s, limited by the SATA3 SSD) or to the normal USB 3.1 Gen1 (450MB/s) port. Both USB cables Type-C to Type-A and Type-C to Type-C are included according to the pictures.