I find that the 120 spool can be attacked with toe nail clippers to take the little 'ridge' off the side of the spool flanges to make it fit 620 cameras.
I have done this by feel in the dark with the film still on the spool, and then load it into the camera, also in the dark. Wind partially, then flick on the safelight to advance it to the arrows, then close the back, unless it is a ruby window one, where just close the back, and wind until you get to '1'.
I have also made the skinny 120 spool work in the 620 side of some cameras by squirting a quick spray of baking release oil (cover all areas you don't want it to hit) to let the glue release from the winding mechanism and lower spring holder.
I then gue tooth picks/wood slivers into the ends of the spool, fill the thing with hot glues, and then stick it into the camera take up holders while the gluie is still hot