It's an alignment issue no question. As to glass carriers, I have only had to resort to double glass with 6x9 cm or larger pano. format on 120 film,because of the thin film base. 6x7cm or smaller is fine with a single glass above the neg : lets face it ; thats the way the camera holds the film..
And that must be some 50mm lens if you are not getting vignetting of the image.