You're going to get lots of flak on here about posting about something that's supposed to be hidden, so have fun with that.
My two cents is that you should consider fitting an actual safe inside so its serves for protection from weather or fire AND thieves instead of just thieves.
Seen some inside old fridges or freezers as well.
Turn a working Coke/Pepsi/whatever machine, into a working ammo dispenser.
....course a burglar would probably just check it for beer.
That is, if you were a burglar....