I thought about making an insert for 35mm and for 120/220 films. I just haven't made it yet but the idea was to use a sheet of delrin. I also thought about using a film holder for a scanner to hold the film flat but that was too thick.
Another thought I had is simply to cut an 8x10 piece of transparency stock and tape the roll film to that. You could tape some film down directly into the holder, but beware that this often leaves a gummy residue for the tape on the backside of your holder.