I have not used Marty McFlyLUT generator.
But here's a shader that can generate any-size LUT. Handy to bake some ReShade effects into one texture: github.com/Fubaxiusz/fubax-shaders/blob/.../Shaders/LUTTools.fx
there are 2 techniques, "DisplayLUT" and "ApplyLUT", you would be interested in DisplayLUT. It will display generated neutral LUT in left top corner, you can adjust size (it represents box dimensions and number of boxes). Then take a screenshot and crop image to LUT size.
But I see the screenshot method more convenient, since you already have the reference image in the background to visualize your tweaks, and still you have to open and edit LUT in some graphics software like photoshop or Gimp to make it do anything.