Because I spend a lot more time on pad stitching the hair canvas on the inside of my kilts I think that really gives much more support both vertically and horizontally to the fell portion, in many ways it become "one piece" of fabric so that it does not shift very much at all. I sew my belt loops on by hand at the bottom through all the layers right through the canvas as well , but then they get fed through the waistband layer and then machine sewn which gives a lot of extra support as well.
There will always be a little movement within any garment, but I think the time honoured method of plenty of pad stitching does help any garment to hold its shape.