Yes, BoingBag3 is meant to add scsi.device to Devs: depending on the type of Amiga, the scsi.device will change accordingly during installation.
This and other rom modules should be loaded with LoadModule that is also included in BB3.
BoingBag3 docs attempt to explain that.
Anyway, if devs:scsi.device is not installed for some weird reason, you can do it manually just by leeching the corresponding file from the BB3 archive at Devs: and properly renaming it to scsi.device.
To make it simple: You say you want it for an A2000, so leech from BoingBag39-3.lha in the drawers \Files2\Devs the file that is named scsi_A600.device, move it to your actual Workbench Devs: folder and rename it to scsi.device. Reboot and now you will have the latest scsi.device for an A2000, version 44.2
Hope it helps!