For a bigger video click here .
Making a few tweeks with the sliders and then baking each, the variations from a screen shot below. Unfortunately I could not find a way to trim in grasshopper so this task is completed once the geometry is baked.
When selecting the point, make sure "point" is selected in Rhino. If "coordinate" is selected you can make a point, but you will not be able to move it around.
Revised file based on some feedback from Gabe. My trim is a bit messy because I could not get it to subtract the correct side. I exploded the surfaces to get the final trim. Here is a link to the grasshopper file.