If you want to keep using a plastic mold you use a release agent or plastic liner. You can use smooth plastic molds without a release a few times but after that it's hit or miss whether your cement recipe releases. The lime in the cement etches the plastic.
You can use the white glue and sand like the other thread you posted on. You can use sand paper to rough up the surface of the ball or just apply the mix in stages. Take a bucket that the ball will sit in and fill the bucket part way with sand or water to keep it from tipping over. Place ball on top of the bucket and cover the top half of the ball with your cement mix. Let this set up and later that day or next day, turn the ball over and finish covering the ball. Blend your edges where the two sections meet. You can also do this in several thin layers and then use a slurry mix to completely cover and blend all layers and edges. It's tricky to cover a ball so take your time and don't get discouraged.

Thank you so much. Your advice is great! Much appreciated.