the length of the feedneck isnt the problem, that would just make it sit higher on the marker, as lemonade said just put tape around the feedneck on the egg to make it fatter so the q-lock grips it. I would also reckomend cutting down the eggs feedneck to lower your profile, to do this mark how much you want to cut off by putting tape around, then start sawing. The tape will also stop the neck cracking as you saw
If you do cut down the neck...try and do it in "one ball width" portions, otherwise you can muck up the eye alignment, so it sees the gap between two balls, rather than the centre of the ball...if you see what I'm saying.