Can I build a portal on top of the Nether in Minecraft Windows 10?
Unfortunately, you cannot build above the nether roof on Bedrock Edition. To do so, you must edit the game code, which I will not go into much detail. However, in Java Edition, the build limit is 256 blocks in the nether.
Putting portals up there is impossible in Bedrock Edition, but phasing/breaking bedrock at spots directly above your portals can help with travel, as you can go to the roof, and run to another destination with ease. I hope this helps.