Okay, today I stared my process to seal the differential case.
I used a counter sinking bit, and followed the guide on youtube. This is the result:
Theres a tiny gap between the o-ring and the case, how ever the o-ring will be slightly pressed with a washer against the case. Hopefully it will seal then.
Since the integy case needs shimming, the seal and a washer will make it tight and nice hopefully.
Tomorrow I will order some teflon washers and 500 000 silicone oil. I dont have any "RTV silicone", how ever I have a can of Permatex liquid rubber seal (form a gasket), and Loctite liquid rubber seal (form a gasket). I think this is equivalent to RTV.
I also have other types of silicones sealants used in motorhomes, I may take a look.