The reflection from the rockface should be quite a bit weaker than the original. It will probably drown in noise is my guess as the surface of the rock is irregular anyway.
Only a plane surface makes a good reflector as otherwise you will have lots of interference. Even if the rockface would be plane it would probably still make a very poor reflector.
A metal surface close to the antenna is tricky though. If you choose the distance wrong the radio waves will cancel each other out.
If you were to block the complete rockface the plate would have to encompass a rather large field of view.
That could mean depending on where the signal comes from the waves will either enhance or cancel out with the reflection.
So just start with a bog standard antenna and don’t worry about reflections i’m not quite sure if you would receive any reflected messages anyway.
One of the messages from the planes is at least 56 microseconds long anyway, in that time radio waves travel 15km anyway, so a reflected message would overlap with the original message anyway.
The worst case scenario with a highly reflective rockface therefore is that the message is not received at all.
If it is received it will be the directly traveling message as the reflected one is a lot weaker.
Hope this makes sense to you