I forget where, but somewhere I read about cutting a strip from a plastic milk jug, stapling it into a hoop and putting it inside the bag neck to keep it open. Works
just like downtown! Experiment with the size of the resulting hoop to get the dimension such that it fits tight inside the neck and it will stay in place. Some folks actually staple it in place, but I haven't had any trouble getting it to stay in place from simple friction.
It gets a little sooty due to its location, but if that bothers you, you can wipe it clean with a paper towel moistened with some solvent. I just throw mine in one of my ammo boxes that I take to the range and it has held its shape just fine. If it does get kinked,
Got Milk?
Here's some images.
Hoot