First thing you have to do is contact TSA and complain about them breaking your lock on an approved strap/lock.
It also sounds like your present case is shot and you should start looking for a new one before your next trip.
If you don't mind the extra two inches, we use the Samsonite Flight with the Bike Fridays. They've never come open during a flight.
Otherwise, you might call J. Gaerlan and use the suitcase he recommends. I recall that it is reasonably priced.