I agree with Redsoxs. A hatchet is easier to control than an axe.
I’m right handed. I have some nails driven into a chopping block. I put the chicken’s neck in the slot formed by the nails. Hold the chicken’s legs with your left hand. Gently pull to snug it up. Swing the hatchet with your right hand. The target does not move.
How wide is the slot? How big are your chickens? I suggest driving the nails to form a V. One size fits all. Make the bottom maybe 3/4“ apart and the top maybe 1-1/2”. That should fit most chickens.
I suggest practice so you can hit the target, like Mr. Hen suggested. Just get used to handling the hatchet.
Thanks, Ridgerunner, excellent description, very complete. I've split a lot of wood so I believe your V design and method will work for me.