Well, the BW image in Plugsnpixels was a PS screendump with all layers open, so seemingly extremely complicated. I now make a few empty layers above a map or a crude drawing of the map layout (may be RGB to easily read the map), with the first layer being 50% black, so that you still see the underlying map. Then on a layer above the black I start drawing in white. Make little black marks in the corners first or you won't copy the whole layer. When satisfied I make a new image, totally black in greyscale tif, same size as the other, name it 'woodmask_terrainX.tif', and copy the transparent layer over that, make it one layer and save. Resize smaller if crude is good enough.
For another mask I open the first one, make it black, copy the next transparent layer over it and save under another name! For me that's a fast workflow.