maybe these two images will help.
i find the effect is MUCH more pronounced when using diffusion heads, or cold light heads.... condensor heads produces an effect that is much less pronounced
mine are filed so i can see clear base film all around the neg.. probably a good 1mm all around..
..the image here was made on a cold light head.
the way ive achieved the 2 solid lines was to make the bottom holder slightly larger than the top.. this matters a lot, because, from what i can tell.. you want the bare shiny metal showing after you file away at the carrier as the more light reflecting off the edge the more youll be able to see it..
i use a variety of bastard files/hand files to achieve this, it takes A LOT of work, at least in my estimation... its hard to file away at these things...
do a buncha test prints along the way to find out what you like, file more as needed...