Krypt - Go to a hardware store and get you some 1/4" or 3/8" vinyl coated cable and some heavy duty crimp ends. About 5' is enough for trees up to about 18" in diameter and its tough to cut through with regular bolt cutters. Then get yourself a heavy lock. You can cover the cable coating with camo tape to hide the shine.
What I do is make a loop on each end of the cable and put 2 good crimps on the fittings.
I had a trail cam stolen on me this year also. Its a shame how people are these days. If its not nailed down they will take it. And a lot of times, if it is nailed down and they can't take it they break it