You could always use an old Apple TV box that you’re not using anymore, but if you don’t have anything on hand, then you should definitely consider the Raspberry Pi case. It’s manufactured by Western Digital’s WDLabs and it’s the perfect size to hold a 3.5-inch HDD. You can comfortably add an HDD to your setup knowing that there’s ample storage for it.
The other option to keep all your components in place would be to hot glue them to your device, or to use a sugru.