Screened bottom board with removable corrugated plastic sheet for varroa detection and to close off the screen when necessary. The front opening is 3/8" high, providing plenty of room from the bees. We've found this smaller opening reduces robbing (though will not eliminate it), and eliminates most mouse damage.
Here in Ohio, we do find it necessary to insert the corrugated plastic sheet during the winter to reduce drafts, but we tend to have cold, windy weather. In other areas beekeepers may leave the sheet out all year long.