Just to follow up on this one: I finally purchased the Digitaliza holders for 35mm and 120. They are excellent tools. Both hold the film very flat, gripped along the whole length of the strip. The mechanism for loading film is brilliant - two plates hold the film flat as the holder is closed, clamping the edges. Then the magnetic plates are easily removed. It only takes seconds to mount the strip, much faster than the standard holders. And because the film is held flat during the mounting by the plates, it remains flat once clamped.
The 35mm holder exposes the full strip of film, including sprocket holes. If you like scanning full frames with film markup and sprockets, this holder makes it easy.
The 120 holder does not appear to reveal as much of the edge, so you might need to resort to taping onto a betterscanning mounting station if you want the full image with edge markings.
Both holders are only a bit wider than the film, so much of the bed of the scanner is exposed to pure light. At least for Silverfast, this makes it impossible for the Silverfast frame finder logic to work properly. It outlines the entire holder as one big frame. To fix that, I put a black sheet around the holder (I just use one of the betterscanning masks - the 120 cutout mask works fine for the 35mm Digitaliza holder).
Regarding shimming: These holders place the film close to the base platen. I roughly measured the distance my carefully adjusted betterscanning holder places the film above the platen, and measured out a block of PostIt note sheets that would raise the Digitaliza film the same distance. Then it was a simple matter of cutting out four small squares from that block of PostIts, one for each Digitaliza corner, and taping them in place with Scotch Magic Tape. The Digitaliza provides plenty of room for these paper shims.
But note, once you have the shims in place, the mechanism for loading film doesn't work properly. The Digitaliza has to sit flat on the table for that mechanism to work properly. My solution? Simply hold the Digitaliza in the palm of my hand while loading a strip - it works perfectly. You could also use a book or block of wood or plastic that can sit inside the shims, allowing the digitaliza to sit flat. That would free both your hands, but truly, this holder is so easy to load you can do it with one hand.
I highly recommend the Digitaliza holders, which you can purchase from lomography.com. They doesn't fully replace my betterscanning holder, but they are now my go to holders for all but my most precision scans. They save a ton of time. And on the v700, you can fit at least two side by side, or even one 35mm and one 120, if you have a mixed set of film strips to scan.