Old thread revival alert....
So up until now, I had been using the small Ilford 3x3 filters in the black frames below the lens, not an issue with any of my enlarging lenses and I keep them fairly close the front. Since I am using a D2 cold light head on my 45MXT, I leave it set closest to the neg most of the time as I mainly print from 6x6, 6x12 and 4x5 negs.
Now I have a rather large 150mm Apo-N that will no longer allow me to swing the filter holder in place. So what I have gone and done is taken a spare Cokin P filter holder used normally for things like split grads and simply thread it on the 67mm front of the lens and then slide in Ilford Multi-grade 3 1/2 x 3 1/2 filters in gently, works great and I only had to trim a little bit off the filters so that they slide in with no binding.
Just an idea if anyone needs one.