I think the trick to avoid the scratches, besides some felt on the edges of the frame (which I've considered but am not sure how much clearance there is between the frame mask and the back which would def scratch it).
I've been trying to do the dual-taut method, which I assume would will for 120 and 135 versions. (I may have talked about this before.)
I load my film, then carefully wind the feed spool to take up slack that may have occurred. This is more common with 35mm film I think. Once it's taut, you know that the take up and the feed are in sync so to speak. Then to advance without dragging the film across the edges of the mask, you just feed the advance as you turn it by turning your source/film side spool. Making sure to do so a little in advance of the speed you are turning the take up. You want to give the takeup/film advance enough to work with but not so much that it hits the back or the mask and slides on it.
If you do this right, your film stays somewhat taut and you have no scratches.
I do think I'd still like to try the felt idea. I need to use a dummy roll and some dummy film and put felt on the edges of the mask to see if that helps. If you put it on the edges as it rounds off on each side, it shouldn't push the film into the back but should keep the film from grinding across that edge.
I don't know how else to work around this minus full on felt on both areas and sanding down the back a tad to compensate for that and I don't want to hack the camera too much.