Rug on carpet slip pad?

I've got a 5x7 area rug laying over my carpet. Inevitably it starts bunching upamd sliding around due to foot traffic. I want to get some kind of pad to lay under it to stop this and I want to make sure that it doesn't damage (buckle, etc) the carpet below.

AI says the best for this situation is felt-on-rubber with rubber facing up. Elsewhere online a pad manufacturer say rubber facing down. Rubber facing up seems to make the most sense to me since it will grip the area rug and be gentle on the carpet below. Any opinions out there?