I used to buy external Seagate HDD’s and pry open the enclosure. Inside it’s just a regular 2,5" HDD with a small sata>usb board. Last time I checked these are still available and might be easier to find (and cheaper). But yeah, you don’t have many models to choose from these days. You could use SSD’s which will save a bit of power but they are vastly more expensive. I don’t think the money you’ll save on power will offset that.