Nice portal that you have here ! 
An additional trick that I could suggest would be to use a non-planar mesh and with a not 2-sided material. For example something a bit curved inward the portal. This way, when the player goes through the portal, he will not pass through the mesh before being teleported.
That’s what they did in the game “Prey” : Prey - Portal detail - YouTube
And also what I did on my own portal system (but I used a cube instead) in UE4 : Vine