I haven’t put one on my acoustic yet, still plan to when I get around to it. I do have one on my Ibanez Electric which allows me to rest my thumb anywhere between, as well as on, the two pickups.
If you search for “ramps” you’ll find a few threads discussing them and their use. Here’s one example:
For my acoustic I plan to construct a wooden one that will span the sound hole and clamp onto the edges of the hole.