I swapped the two SelColors so that the solid line is black and the dashed line is white instead of the other way around, which helps in my case. I may experiment with other colors. Setting pen width to 2 instead of 1 also helps, but that required modifying your code which I don't like to do.
Update: I ended up modifying your code to set the Pen width to 2, solid as black and dashed as white, and instead of psDash I went with psDashDot for both the main polygon and guides.
If you have the chance, it would be nice to be able to set these (pen colors, width and style) via some sort of additional TImageEnView.CropTool properties so I don't have to modify your code. :)