Since MINECON has announced the incoming update for the nether, you shall be prepared to start the journey a bit ahead, maybe with fancy redstone technology. Here is the completely hidden stylish portal in a simple redstone design, but this one has EXTREME SPEED! Got fastest possible opening / closing as well as MOST COMPACT size for a 3x3 hidden portal, IMO, and any challenge is accepted! (beat me if you can :p)
It is kinda perfectly tuned with some ingenious coincidences, except you may hate observer block because people could think it makes things lots easier and with less logical thinking. I personally joy to tears of welcoming it after returned to play the game for like 5 years as IMO it WAS one of the greatest updates in redstone history (I know i missed lots of things), and I have already updated everything in my base with it.
How to import by structure blocks (in vanilla MC):
(Please load it 2 times after some seconds if you see water leaks, it is a bug in MC 1.14)
~ The actual portal has even less blocks on the top than that in the screenshot. You can extend it to 4x3 (nx3) as well as make it 2x3 with -1 width, If you tweak a bit circuit at top / bottom. It is very easy and I'm sure you can do this.
~ Thanks to BSL shaders and Soartex resource pack for the amazing screenshots.
~ Please enjoy my creation and have fun with it in your games!