I think that until the chickens are trained, you are going to lose some feed to the critters you wanted to avoid feeding in the first place. In the long run, you'll save money. I started training the chickens on my home-made treadle feeder by propping open the lid for a week (by placing a brick on the treadle) just to get the birds accustomed to stepping on the treadle and eating from the feeder. I stood two cement blocks at the sides of the feeder to limit access from there and force them to step on the treadle. During the second week I propped the cover open so that it opened or closed only about 1" when the chickens stepped on or off the treadle. That got them used to to the movement of the cover mechanism. I then propped the cover 1/2 way open so the chickens could see the food, but had to step on the treadle to access the food. I only propped the feeders open for brief periods of time, usually about 20-30 min in the morning and late afternoon (and anytime I was around to observe). The rest of the time the chickens were able to step on the treadle and open the cover and access it. Two of my six birds learned within days to fully use the feeder at will. They would calmly step up and open the feeder and eat. The others then would move in as three can easily eat at once. Interestingly, not all birds learned to open the feeder right away--the birds at the bottom of the pecking order learned first. In fact, my hen at the top of the pecking order didn't open the feeder on her own for nearly a full year! She always walked up to the feeder with one of her buddies and they opened it for her and she would step on and eat. Then one day, while broody and in a mad dash to eat, drink and get back onto the eggs, I saw her walk right up and open the feeder on her own. It demonstrated that she knew how to open it, but preferred to have another open it for her. Now that she's done being broody, she opens it all the time on her own. I suspect that training would be easier on birds that do not free-range. My birds free range a lot and don't eat much of their commercial feed in the summer, so the motivation and opportunity for interacting with the feeder are lower. I wouldn't worry at all about another hen feeding and getting her head stuck when one steps off. I've never seen it happen in a year of use now.