I still struggle with the IDMasks and setting everything up in the Compositing section of Blender. So, I will have to keep doing things like this until my mind learns what is going on.
Here is an attempt to show a UFO landed in a walking park near me. I struggled with finding a way to get reflections of the actual ground beneath the UFO to show on the bottom of it. I Think I accomplished that. I gave the ground plane in the scene a UV mapped texture of the image that is the background. I then scaled the image so only the grassy area that is shown under the UFO is on the textured ground right under it. It seems to have worked but I kept the hull’s reflection low so as to make sure it didn’t look too obvious that the ground texture was stretched.