You put 4 separeate Trigger Boxes on each side stretch them out until they overlap. I also just noticed you need 4 Copys not 3 if you want Up-Down-Left-Right looping (think of the Corners)
And hey you are right thinking of it a bit more you need the Screensize(playable area) at least to calculate the Teleport Location. If you using a Orthograpic Camera thats no big deal you can read out the Viewport Size directly. If you using a Normal Camera you can get away by moving the Camera closer or farther away in most cases. Depending on the Ressolution. Otherwise you need some additional math to calculate the Frustum.
As far the other stuff goes it stays the same.