I don't have a photo either. Think 5" to 6" nails with a separation of 3/4" at the bottom and 1-1/4" to 1-1/2" separation at the top. Don't obsess over the top measurement, you don't have to be that precise. Put the nails to one side of the stump as shown in that photo so you have the width of the stump to lay the chicken out.The v-shape you describe may be the key, but I’m having trouble visualizing it.
I hold the chicken by the feet and let it settle when I lay it down. Hook the head in that Vee and gently stretch the chicken out. You do not need to pull tight, you just want to hold it steady. I've used this for a fairly small hen to a heritage sized tom turkey.