The cube port on the back is for the oddball blue tipped USB3 connector on the loopback cable and next to it is the SATA power connector making it a powered hub.
That's a full-sized USB 3.0 Type A connector.
Seeing this full sized image, you are probably meant to connect this thing to a back port on your machine, rather than to a motherboard connector, or perhaps: you can do either way. Most modern PCs have fairly large holes in them with rubber grommets through which you can route a variety of cabling.
I'm surprised you haven't installed an LS-120 behind your Zip bezel, doesn't it line up well enough?
I never had an internal one, and to be perfectly honest, I don't know if trying to "make it fit" would have occurred to me. If I remember correctly, the Zip bezel is separable from the CD bezel in that era, so if I did have a working internal LS-120 drive for that machine, I would probably have just left off the bezel entirely.
Largely, this falls under the category of my advice where, whenever possible, I tend to recommend that most people avoid starting collections of vintage media for no good reason. This applies here, too, to be honest, you "could" put an LS-120 drive in a G3, I wouldn't do it unless you were using floppies extensively for some reason, or you already had an LS-120 ecosystem built up. Notably, I, personally,
am engaged in building out a collection of different removable media, because the topic and the technologies interest me extensively, and I still haven't gotten any of my LS-120 drives working because I haven't bothered to buy the special cleaning disk, which you need to use the drive at all, because normal cleaning floppies will not work in those drives.
Plus, even if I had LS-120 working at the moment, on the Mac, the "authentic" LS-120 experience is buying one for an iMac because it's 1998 and LS-120 and a plain USB floppy are like a $10 difference. That particular blip, coupled with LS-120 actually shipping earlier than USB-based Zip drives, is Imation's main claim to fame. I think there were some PowerBook bay modules as well, but by volume, that's an extreme minority of the LS-120 drives that existed, I imagine.
USB floppy drives are that much better because, as I mentioned, they're easy to clean and they're generally still compatible with diskcopy6, and are still suitable for writing diskettes for a CC, but, if you have a network, I wouldn't even bother with that.
If someone wants to fill the void in a machine, the best way to do it is to place a Zip drive there and leave it unconnected, or if you have use for it/them, put a hard disk in that spot.
If someone just wants to know what'll go there, ultimately the answer is "not much that will do you any good."
Notably, the internal USB hub that's linked is like $60 and for that money you can either buy three or four of the USB 2.0 4-port hubs (powered) from Best Buy or you can buy a better external hub with a lot more ports. You could probably buy a better hub
and a Zip100 drive to fill the void for $60, so which direction any particular person goes will depend a lot on their preferences.
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