On the Meopta for 35mm simply use the plate which goes on the bottom of the glassless neg-carrier as normal, then the anti-newton glass on top. The lower plate has a slightly larger aperture in and the glass has no aperture at all of course. Spares for the Meopta neg carriers etc. are still available.
Alternatively, use the double-glass carrier and mask off as required with the built-in sliding masking-arms.
If you just want a black line instead of the "edge of the negative" look, then make a black card a few millimeters smaller than the enlarging-easel format, place it in one corner of the paper/easel, expose for black, move to the diagonally opposite corner and expose again. That gives you a neat black border which will be surrounded by the usual white edges. Don't forget to make your print exposure too of course.