Instead of locking up the rifle it might be easier to keep the ammo locked up instead. Most of those inexpensive plastic "clamshell" gun cases have the provision for a lock but any motivated youngster could easily get a rifle out of one of those, and back in, and you'd never know. Don't ask me why I know that
Maybe get a steel ammo box from the local army/navy store and put a good hasp and padlock on it. That would keep the little suckers out. Maybe even bolt it to the floor so no one else can get it out, either. Good place for valuables while you're away from the bus too.