I use this method on 120 - and 35mm. In full illumination, a frame down from the negative of interest, push two push pins through the film to have the unsharp mask made if it. (Trim back any molding ridges from the undersides of the pins to ensure that they are not a surface that could scratch the emulsion, and work wering cotton gloves to keep fingerpints off the film. Pierce the procesed piece of film now as well if you use it (I haven't to date, but will try it now)

Use a black foam core board piece for the baseboard.

Under safelight conditions push the pins into the lith film and into the foamcore basebard ( I use a half sheet of 4x5 for each mask, since smaller sizes are a beast to handle when developing.) Top with the weigh down glass, and diffusion material, and expose. Pull the pins after exposure, to process the unsharp mask.

After processing I trim the lith film to the width dimension of the negative carrier, and re-register the original neg with the unsharp mask by matching up the existing pin holes. Usually the pins hang beyond the end of the neg carrier, and I just push them into a small piece of foam core a half inch wide by film width tall. If the pins interfere with the neg carrier, I tape, and pull the pins.

35mm is a challenge to realign, due to the curl of the film. Pin register, and flatten between two weight glass peices over a light table surface with only an edge hanging out where the two bases are to be taped together. Then re-shift the weigh glass pair to expose the other edge for taping.

So pin registration is available for almost zero dollars with this method.