Sounds like you want something:
http://www.rackmountsolutions.net/Security%20Doors.aspI wouldn't recomend thier use with servers, as I'd imagine it would limit airflow fairly severly. They're more designed for protecting knobs from people adjusting them.
What you probably should have gotten was a server with a locking bezel. Most of Dell's 1U server have a big metal bar that's designed to keep fingers away from things, but still alowing airflow.
Honestly, at this point you best bet is a 'don't touch' postit note, and a webcam to show you who touched something if you have problems.