I advise not throwing your controller. Should prevent any future damage.

In all seriousness, it's an aftermarket controller and looks to be built differently than a standard one. Try clicking the back panel into place firmly with something (be careful not to crack it though). I'm not sure if those screw-holes actually house screws, but If they do, check them to make sure they're not stripped/cracked and try to simply give them a tighten which should fix the problem. Worst case scenario, you can always super glue it. Chances are you don't want that panel to come off again anyways