What are you using for the post? If it is a yarn of some type just leave it long and hold onto it with your off-hand while you wrap down with the hackle. You can then pull it apart wide to further force the hackle wraps together--and then trim it to length.

Am I correct to assume you have secured the hackle to the post and not just simply the thorax? If not, you might try that--the stem itself should make a rigid enough post for you.