[BDM]

Here's an idea. All of mine move side to side about 1/10", maybe a little more. But, if it moves, try this. Cut thin fine strips of tape, just enough to place along the edges. This will help keep it tight. It doesn't have to be exactly as I've stated, but anything similar to achieve the same result. Keep in mind the shadow mask does bulge out just a little.

Second, the dial disk shouldn't rub against it. There is an inset screw in the dial disk mount(the dial scale mounts with a retaining clip). You can loosen it, and move the dial mount out just far enough to avoid the rubbing. Also, the two upper tabs that the dial scale sits behind, may need to be bent out slightly to keep things in alignment. That was the dial no longer rubs the shadow mask, causing it to move.

Last, make sure if you move the dial disk out, to re-align the dial scale so your tracking remains proper.

Does this help?