I have seen some full-coverage hard plastic scabbards for mounting on ATV's or snowmobiles that could probably be adapted pretty easily. Have a look at the Cabela's or Gander Outdoors websites.
Another thought is police supply companies. We always used locking racks mounted to the floorboard of the cruiser to secure a pump 12-gauge shotgun. These typically lock up at the rear of the slide to prevent anyone from inadvertently chambering a round while in the rack. Some have key-locks, some use electronic locks. Might require making a bracket to mount it to your tractor fender.
As far as safety goes, we always referred to a shotgun as "cruiser ready" when the chamber was empty, magazine full, action cocked, safety on. To make the shotgun ready requires hitting the slide release, racking the slide to chamber a round, then disengaging the safety, then the shotgun is ready to fire. For an experienced person that all requires only a few seconds, but the shotgun is always safe to transport and handle without undue concern.