When posting a quick tip about this topic several weeks ago, I wasn't very specific about how to setup the material at all... so here's the missing addendum.
When comparing the screenshots below you can clearly tell that my simplified version is not physically accurate, as it's missing a fesnel effect which would strengthen reflections at extreme angles. In addition it doesn't take MaterialAO and Specularity into account.
You can however add these missing features if you need them. For further reference you might want to take a look at the PostProcessAmbient.usf shader file to see how epic did it.
Direct Light Only
Built-in Ambient Cubemap PP
Custom Ambient Cubemap PP