I didn't use the glass fix. I tried but it is difficult to fix properly. If the glass is added then the back won't fit on. The plates attached to the back of the camera touch the glass and prevent it from closing. To do it I'd have to fit the glass so it fits right in the opening of the camera body. Nothing to support it there.
I guess I could have removed the film plates from the back door but than I'd have a bunch of rivet holes to deal with. Instead I added some foam strips to the door. Between the foam and the tightness of the film roll there doesn't seem to be any flatness issues.
I have only shot a couple rolls however and they have all been in bright sun at f16 or smaller.
If I get any problems using a wider opening I might try the glass approach or fashion some sort of supporting strips just on the edges.