Create a new image in GIMP.
Set the Width and Height to the size of the panoramas: Width: 8192 Height: 4096.
In Advanced Options, set the Color space to "Grayscale".
Set Fill with to "White".
In the new image, select the regions to mask out. Use the "Free Select Tool" to define a polygon. You can select multiple regions.
Set the foreground (FG) color to black (000000).
Right-click on the selected region, then choose Edit -> Fill with FG color.
Export the image as a PNG file.
Use the custom operator mask in post-processing by adding the file to your SiteMaker project.
Note: The pixels to mask must be set to zero! Pixels with any other value are not masked out.