repost because I forgot to mention, figured out my film holder flatness issue
guess the guy who makes these doesn't shoot a lot of 35mm, because the track was designed for the exact width of the film
the 120 holder on the other hand is about 1mm over-sized on the width
this means the little errors in 3D printing the 35 holder pinches the film and causes it to buckle
I just took an xacto chisel blade and shaved the bumps off, then finished with high-grit sandpaper, no more issues