Well in Source Engine the portals are 2d planes so as said, higher FOV 2d capture actor should do exactly what you want.
Unless you have 3D portal geometry that needs to show other dimensions, you should be able to get by with 2d capture. Even then you can screen project 2d capture over 3d portal geometry and still get decent result.
Maybe if you really need that 3d capture cube, you can update it’s sides one per frame. This way you’ll have new cube capture every 6th frame and you can blend it with the previous one. Choppy, but still a cube.