Anders,
I've just checked my CLS450 head and you're right the white perspex of the diffusion box is near 14 mm out of the mounting surface of the head.
This impose the use of a TAUNEG neg carrier instead of an OTONEG (be it model I or II) because the OTONEG has a ridge running on the large side of it, at the rear which will block movement when touching the protruding mixing box. The TAUNEG use a different glass fixation and does not have the ridge.
I own a modified OTONEG where this ridge has been cut off and filled. IT has been done poorly but the neg. carrier is still functional.
On the other hand, the TAUCOLI cold light head does not protrude into the neg carrier stage. So one can use the OTONEG with the TAUCOLI.
IMHO, if you only own an OTONEG, you should make a mock up using cardboard or perspex with the surface of the mixing box just flush with the negative stage and see how it comes.... I wonder if it will make any difference (except for light leask you would have to seal)