To do it the same way as it is done on the exmaple picture, you should have used much thicker stone wall and use in-place generic model to carve out the stones from both sides in different way and create a pocket in the middle to place the CW in. would look much better
Or you can do the same thing but instead of changing wall's profile use void form of model in-place. Fiddling with wall's profile may cause a lot of bugs if it needs to be moved or changed in height. Anyways, Great work!