We ended up using the killing cones I made for the roosters, they worked OK for the smaller birds but I had to grab them to control the wings while the death throes were going on. Tried the bag on one of them but it was too big, she just flopped around inside it. Should have used the bag on the tom, his wings got loose and whacked me pretty good.
I ran a rope over a rafter in the greenhouse, with a loop in the end. Catch a bird, hang it upside down, tie its feet together, slip the noose over the feet and hang it in the cone. That way it can't kick or get anywhere. Pull the head out, cut throat, lay knife down (very important!), and grab the bird to control the wings. The bottom of the cone extends into a bucket, so nearly all the blood is contained.
On bigger birds I think the feed bag trick would be better. Maybe next year.
We had a neighbor and her (college freshman) daughter come over to help do their bird. They actually enjoyed it, the daughter is really interested in food production and wanted to experience the whole deal. She was kind of disappointed I wouldn't let her wield the knife, and was fascinated by the evisceration process. Squeals of delight when I opened up a gizzard and showed the liner, etc. Kind of fun when you have enthusiastic youngsters around!