If the Proxars aren't designed for your camera and lens, I can't see how they would work. Those are very specific in their designs. Why shoot close ups w/ a Bessa II though? I've owned several, they have great lenses, I'd just crop the neg, that's a huge one you've got.
TLRs are easy to shoot w/ close up lens attachments because they're almost like a SLR. You actually see the image go in and out of focus visually, while w/ a folder you don't see anything happening, you just see a static view. Maybe I'm wrong, but shooting closeups w/ aux, close up lenses that are mounted on a 120 folder seems like a case of the wrong tool for the job.