What are the best HDD backup options for an OS9 tower?

I don't assume I can simply drop four drives in the box and setup some kind of RAID array . . .

I never thought about backups back then, and since the advent of Time Machine, I've always used that.

What do you all suggest?

Ideally, the solution would not require any peripheral devices. I do have multiple drives on hand to use internally. Up to four, but not all of the same size.