based on 50+ years of having chickens and livestock.. knowing people who had similar experience to find out that the pred was caged in with the birds.
based on the OP saying every opening was covered or boarded up..
based on a weasel does not eat that much of a chicken.
a raccoon would have eaten the whole thing being that small.
still sounds like a rat, to me.
I must have missed your post of what you think it could be.
If you suspect where it is getting in,
or on the inside, where the chickens don't walk,
lay down a good dusting of all purpose flour.
it should show the tracks as if it was snow.
based on several years of trapping..
.........jiminwisc....