Starry Landscape Stacker can work with masks that are created in advance with an image editing tool rather than generating its own. You can construct a mask with any image processing tool (e.g., Photoshop, Pixelmator Pro, Affinity Photo) and supply the mask to Starry Landscape Stacker along with the light frames and dark frames. The mask must have the same pixel dimensions as the other frames and must be stored as a TIFF. It can be a greyscale image with a non-zero value for sky and 0 for ground or it can be an RGBA image with the mask in the alpha channel. The RGB data will be ignored. If the alpha channel is all 0 or all 1, it will be ignored (because ON1 always includes an alpha channel that is all 1). If you want to make a mask that covers the whole image fill an image with white and export it as a greyscale image.