Existing SCSI2SDs are internal devices - you can put them in a case (such as the one from a SyQuest drive). There is a new version designed with a DB25 connector directly on the board designed to connect directly to the external port.
More information about the external-only variation is here:
If you wanted to use an external-only SCSI2SD with the Mac Plus, which does not provide termination power, which is how the SCSI2SD can be powered on newer Macs, you could power it off of a USB phone or tablet charger and a Micro USB B cord.