The hive box should tilt a tiny bit forwad, so that dew, rain, etc runs out the front entrance. Otherwise, filthy water sits inside the hive - bad for the bees and rots (or in your case rusts) your box. It seems that moist hives also contribute to chalk brood. I also think it makes it a tiny bit easier for them to be good housekeepers as they can roll their rubbish out down hill ;)
Wouldn't mind seeing a pic of your metal base hive - only every seen wood and weathertex before.