It’s meant to be right behind/in the big rectangular hole. It should have some screw holes on the side where it connects to the outlet (likely also has a power switch) that match up with the ones in the case. It looks like there’s an optical drive in the way though. If you like I can try my best to circle them for you.
I’d love (hate) to see the computer with the side panel open.
If it works don’t fix it. Only issue might be extra dust but it sounds like there’s no solving that issue in your computer.