If it's one of the ravens you buy at the store with the wires extending from the feet, then just carefully insert the wires into the foam. It won't damage it and when you remove the raven, the holes left by the wires are barely, if at all noticeable. They can also be filled with a dab of joint compound or even paint if you want. I have ravens in a few of my stones and it's never damaged them. hth.
If it does not have wire through the feet you can use a stiff wire bent into a U shape to pin it's feet to the stone as long as it's not really heavy. Any unwanted holes left in the stone can be covered with a dab of caulk then painted with a dab of grey.