Is this what you mean?
(added a wiretex around the portal box to illustrate better)
This is in max, but it's all material properties so hopefully works just the same in C4D:
1. portal box one side deleted, give this a material with full refraction at IoR 1.0
2. objects inside box have a material with the following:
- rayswitch map into opacity slot with color set to black for refraction, the rest white
- rayswitch map into refraction slot with color set to white for global illumination, rest black
If you don't set rayswitch in refraction slot then you get shadows from your scene showing up in the portal box when it's 'empty', this is the bit that's tricky.