I have a treadle feeder similar in design to those posted links and whilst it does keep the wild birds off, it took the grey squirrel less than 24hrs to figure out that he could stand on the treadle and push the flap up to get in - so not vermin proof IMHO.
I did manage to deter mr squirrel by electrically isolating the treadle from the body of the feeder and connecting to an electric fence energiser (whilst out of reach of the hens of course) - but that only works until the next squirrel comes along.
Mine is 10-12kg I think, but as I've only got 2 hens at the mo' I tend not to fill it anyway as it takes them a while to eat that much.