ofn-layer-tiles requires the image size an exact multiple of the tile size, so (depending on image ppi) the image needs padding out. A bit of arithmetic and you can use Image -> Canvas size for that.
On the other hand, there is https://posterazor.sourceforge.io/ There is a OSX version (might work)
On the other hand, there is https://posterazor.sourceforge.io/ There is a OSX version (might work)