After 3 attempts, first in Moi then 3dsmax and finally in Moi again I managed to get the model right.
I accidentially forgot to shell the transperant cover, before I boolean'd all the details out.
So i faked it in 3dsmax during render as thin walled material.
It could have looked better, but happilly it will not be as big in the final render.
Start to love Moi even more, day by day.