I still stand by my original post on the subject, If we are going to keep putting money in a box in the middle of nowhere, it will continue to get broken into / nicked. No matter how strong it is. They'll always have a go.
I know when there where repeated car break ins at Stanage Plantation car park, and at Curbar Gap car park, the authories set up a little sting operation with hidden cameras.
As the breakins where almost daily, it didnt take long to capture the culprits in film.
Problem here is that its every 2 or 3 months, so it could be a while.
Still - Im sure its not beyond some of the technical boffins on here to come up with a little camera that could be buried in one of the walls near by pointing at the moneybox. Linked to a sensor in the box, (motion sensor. microswitch or somesort). door comes off, camera turns on. bingo.