Yeah, I doubt if any treadle feeder is rabbit proof, they have the weight and the reach to use it. To be fair a chicken feeder is expected to be in a coop. Asking a treadle feeder to eliminate all feed theft is not a valid complaint no matter who makes the feeder.
To the OP, is it the Grandpa feeder, AKA the Granpa Xi feeder? You said you had to block it open so I assumed it was that or one of the other Chinese made clones of the Granda Xi feeder. If so that is a huge flaw in their design, requiring the feeder to be blocked open because as one poster has already mentioned it teaches the birds that the lid isn't supposed to move when they use the feeder.
The other issue is the free range. If you are expecting a chicken to learn to use a treadle feeder they HAVE to be cooped up with NO other feed available. Including old feed in any deep litter, snacks, free range, nothing but the feed in the feeder. The hens HAVE to get hungry so they are motivated enough to overcome their fear of the shiny contraption that has invaded their space. A half day is plenty for most flocks, one bird will learn and teach the others.m Occasionally there will be bullying issues, so if that happens remove the bully for a few days so the smart hen can teach a few more hens. Usually the smart hen learns in a few minutes or hours, others gang up on the feeder at first then figure out how to operate it themselves.
No matter the manufacture of the feeder make sure it is sitting solidly on the ground and fastened to a post, a wall, or to a big chunk of plywood staked to the ground. If the feeder is wobbly the hens won't trust stepping on it. A lot of the Chinese made feeders like the Granda feeders are stable when full of food but less so when half empty so find a way to secure it. Make sure the treadle actually bottoms out on something solid too. And have someway of forcing the hens to come in from the front, gallon jugs of water, cement blocks, a chunk of firewood.