The main cause of smell (other than poor sanitation) is DAMPNESS. The way to minimize chances of smell and flies is to do everything you can to keep your run very, very dry. This is much more effectively done by good engineering and design than by planning to rely on stall powders and such, although powders can occasionally be useful in a patch of unusually difficult weather. See my "muddy run" page, link in .sig below, for a lot of things you can do to build a run that will stay as dry as possible.
Good luck, have fun,
Pat
Good luck, have fun,
Pat