I don't have the patience for graphics programs, so I'll just try to describe what I think should be some basic requirements. I'll leave the art to you.
First, you want a straight string pull. That is, there should be no angling off to the sides on the headstock side of the nut. You're begging for tuning problems otherwise.
You're almost certainly going to be threading the strings from the back of the headstock through ferrules, like you see on the back of a Tele. You'll also have smaller ferrules on the top, so the string has something solid to bear against. So, you can place those as you will. A "V" would work, or an angled line, or a line parallel to the nut. Have your way with that, as long as the holes line up with the slots in the nut.
It seems like a neck without an end barrier would feel weird. Like without some tactile indication of where you are, you'd lose track. I don't know; I've never played a headless neck. But, if that's so, then you'd almost need a sorta small "T" up there, so you could maintain the feel of having a headstock without one actually being there. Maybe as if you took a regular headstock and just took a chopsaw after 80% of it. Round off the edges, of course, so you don't take somebody's eye out. Or not, if you play places where traditional weapons aren't allowed...
Just some thoughts.
