ondeko is right. Chicken wire and monster mud was the easy solution for me. In regards to the reaper tombstone I did, I got a cheap Michael's hanging reaper and dunked the whole thing in the monster mud, removed the excess Monster Mud and screwed it onto the tombstone. Let dry and painted it a couple of times with gray Drylok to make it waterproof. (
See my Beloved tutorial for more detailed instructions) It's been in the rain and sun and no degradation at all. Here's a pic of the reaper right after it was monster mudded:
Here is the back after I gave it two coats of Drylok. You can see it is starting to take on the look of carved stone:
Here's is the Reaper next to the Beloved: