On the bottom surface of that cage, there should be some holes that line up with the screw holes on the drive.
Or, there would be 2x arm things, that screw to the side of the drive and slide into the grooves on that cage.
Said arm things are specific to that cage.
Looks similar to my case. Which comes with little "sledges" that the drive is to be mounted in and that slide into the cage. I've seen others that instead use little brackets that get mounted at the side of the drive.
The drive basically sits where the case is solid at the side, the little hole at the front so that the sledge or brackets have a retention point to prevent it from sliding out on its own while moving.
So try finding the sledge or brackets that came with your case. If you don't have them, might be worth buying replacement parts. Or making a solution yourself. If looks aren't important two pieces of wood with screwholes should do, or some pieces of sheet metal.