been messing with this all day. tried everything to get it working nicely and finally got something. I’m using an app, Eevo on android, but i think it would work on Youtube and Cardboard:
I used the top bottom camera and rendered it out at 3840x3840 as a png sequence. I took that in AfterEffects and put it in a 3840x2160 comp and stretched it so it fit perfect. When I opened it on my phone it looked great. I could see just one subtle seam but no black holes on top or bottom.