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!
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.
And, you may wanna replace some glasses with your own favorite blocks to make it looks hidden, OFC.
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!