How to render cubemap as post process material?

Hi, I am trying to render stars as a post process blendable material as described here:

Unfortunately there were not enough frames in the video to see what nodes he used but I came up with this:

I didn’t need all that stuff he talked about in the video to adjust the material when in outer space since my game doesn’t have the ability to move from the ground to outer-space. I just want to show stars when the sun drops below the horizon.

Unfortunately this setup produces some bad ‘flickering’ artifacts around the edges of the objects in the foreground an the background. I can’t really describe it any better than that but it is very noticeable in game. Also I noticed that if I use the epic quad default engine cubemap it doesn’t render correctly at all for example:

tldr: What is wrong with my post process material graph? How do I property render and blend cubemaps as a post process material?

By the way, full material showed in this video: From Ants To Outer Space | Inside Unreal - YouTube

But the stars are visible on the horizon during the day. How to get rid of it?