The problem

I wanted so there are no boundaries on the above dimension, for example, I want to add aether dimension above the overworld, but I can't go since there's no portal block that transfers me into Aether seamlessly.

Objectively, I want to make it like:
https://www.curseforge.com/minecraft/mc-mods/immersive-portals-for-forge
But this mod is for 1.14.4 and my mods are 1.12.2

My solution

A possible solution is to detect or place a seamless invisible portal block in y=0 and y=256, and also removing the bedrock from BOP (Biome O Plenty). Or replace the Minecraft bedrock into a portal block.

Why the existing features won't cut it

No seamless and possible way to access Aether from flying up in any way. No easy way to configure it either.