I want to expand on your answer, as it took quite some work to get it working with my materials. I needed to set custom cubemaps, for materials that have various metal values, and to blend with the background. Here is my final script for the reflections.
https://puu.sh/zVrS8/33308792f0.png
And finally, before going to the emissive channel, it adds to the emissive color I already have.
https://puu.sh/zVrT3/9c5a8611ed.png
This is what my crate with standard ue4 reflections look like.
Editor.
https://puu.sh/zVrUZ/6649b65e9d.jpg
In Game
And this is what the manual cubemap looks like on the same model.
Editor.
https://puu.sh/zVrVN/2d7183b576.jpg
In Game (Sorry for the tiny screen).
https://puu.sh/zVrZk/0059ffea7b.jpg
It’s not perfect, but it’s way miles better than having a black reflection before due to the procedural generation. It blends quite nicely and looks natural. Before: