How about something like one of those clear plastic boxes that you see over thermostats? I found this manufacturer. Their stuff seems a little expensive, but maybe you can find something cheaper.

Also, search keywords I can think of would be "tamper", "tamperproof", "anti-tamper", "enclosure", "cage", "deterrent".

Honestly, any sort of box would do. Maybe a medicine cabinet? See if an army surplus store has some sort of ammo boxes?