Another very simple mitigation for the head/carrier interface is to go a hobby shop and get a sheet of adhesive-backed black felt. (It's not real felt these days, but just as good for this use.) Then use your trimmer to cut a long strip about 1/8th inch wide. Take that strip and stick it all the way around the underside rim of the condenser housing (this is also where the round cold light cans reside).

The effect is that when you lower the head directly onto the carrier top, the felt seals up the slightly imperfect fit very effectively. On my D5XL there is absolutely no leakage from this location. And a nice side effect is that if you're equipment-anal like me, the felt prevents any marking up of your nice clean carrier tops, especially if they're white enameled.

And again for the inverse problem, I've used this same felt to seal up the interface between the camera body and bellows mounting frame on my 8x10 (a like-new restored Calumet C1). Had light leaks before. Has absolutely none now.