Well, for perspective we've had a suspected rock on our engine splash guard/skid plate/plastic thingy underneath the engine for over two years now. So far we haven't notice any issues with it bouncing and sliding around down there.
That said, if you drive on enough dirt roads it'll fall out by itself.
Also if you get an oil change and mention it to the oil change place they may be able to retrieve it for you.
If you really want to tray to get it yourself, decide where you can access the holes in the plate the easiest, then accelerate forward an brake to move it to the front, or backward and brake to move it to the back, etc. Then you can use a magnetic retrieval tool to pull it out.