Only the stock extended battery door will offer you any meaningful assurance that it is really water resistant. Third party battery doors may have the gasket but how can you discern their quality control i.e. whether the gasket really works as promised? You can't unless it's a major brand name vendor whom you trust when they say that their products meet specs.Or if some trustworthy third party undertakes to test non-OEM parts.
If you want to gamble, Ebay, et al have lots of Chinese battery doors which may or may not actually seal against water.