Oxygen,
Shader Settings from Q3map2 manual(link below)
*portal
Specifies that this texture is the surface for a portal or mirror. In the game map, a portal entity must be placed directly in front of the texture (within 64 game units). All this does is set “sort portal”, so it isn’t needed if you specify that explicitly.
Make a shader and apply it to the area needed, place a misc_portal_surface within the 64 unit limit. It is easier than you may think. You should take a look at the whole shader content list. HERE
-cheers
O-yeah, no camera is needed, but you never know what the person is truly trying to acheive. Play around with it.