You're probably already doing about the best thing that you can to secure them. I have one of those big wrap-around porches and my bedroom is right next to where I occasionally put a bear. In my case, I feel pretty secure since I can hear even the lightest foot step on that wooden plank porch and my nearest neighbor has an eagles-eye site on the hill about 200 yards away. Says that he can bead-in, dead-on anything he wants at that vantage point.
Personally, if I cared a lot about such a carving (and I do) then I only put them on the porch if they're light enough to set inside when away for a period of time. If it weighs 500 + pounds then I'd say it's probably gonna stay there unless a RR track runs by your house!
Good luck!