FWIW, after a bunch of prints with distinct double images, I started simply hand holding the filter material under the lens for split filtering with my D2 and cold light. Just lifting the head enough to slip the filter in on top of the negative carrier would (sometimes) move the carrier enough to make a visible double or a much subtler blur.
This won't work well when I'm ready to dodge and burn, however (one hand to hold the filter, one to operate the switch, doesn't leave anything to dodge and burn with), so I'll probably wind up fabricating a simple frame that will fasten to the bracket for under-lens multicontrast filters (thoughtfully provided by Omega on each lens board and cone), to hold the 6x6 (or smaller, if I cut some smaller ones) filters. Then, of course, instead of just moving the filter to avoid projecting dust and scratches, I'll have to find a way to keep the filters clean and scratch free...