This uses plastic slide brackets alright, probably just not the ones you have. Those things are horribly non-standardized, so chances of finding a matching set would be slim indeed. Do you have a brand/model name for the case? Or for the PC as a whole? It might be possible to find something based on that.

Yep you need drive rails.

3D printers are your friend

there's always a way. personally I would cut some rear expansion slot covers down to smaller rectangle shaped size. Go to the hardware store and find some longer screws. drill a hole in the rectangle metal plates I made. put the screw in the hole I made in the plates. screw the plates in from the outside of the bay so it holds the drive in place. Torq the screw in so it sandwiches the plates against the bay and pulls the drive tight to the inside of the bay.

Alternatively you could do this with just one longer plate and drill two holes in it and attach it horizontally instead of two smaller ones vertically.

Plasma wrote on 2022-02-09, 00:51:

Yep you need drive rails.

First thing that came to my mind when I saw the bottom 2 pics.

@Piggy, if you want to know what they look like, you can google something like "drive rails atx" or "drive rails pc case" for instance.
They can look quite different from one case to the next, these weren't really standardized. So you probably can't use ones from a different case.

But even without 3D printing new rails, you have holes so the FDD can still be attached but you'd need to juryrig something together. Alternatively you seem to be able to mount optical drives so one other option could be to use a 3 1/2in to 5 1/4in adapter and mount them in a 5 1/4in external pc case bay (this would be my preferred solution if I want the drive to be firmly attached).
Another possible solution could be to fill in the space between the drive and drive bay with some stuff so the drive remains in place, but this may be less secure.

Be creative. Or manage to find the original design drive rails somewhere.

Thanks everyone, finding any info about this case is extremely unlikely, so I will need to find an equivalent or 3d print something, but confirming that I needed rails helps a lot. I tried googling many things with no luck, but looking up "drive rails at case" returns some very promising results. "Rails" was the word I was looking for, I was searching for brackets.

And yeah, I gotta get a 3d printer one of these days, it's starting to become more and more of a necessity the deeper I get into the retro hardware hole.

LGR just cut some rails by hand out of a sheet of plastic in one of his videos, seemed to work well enough