12x12 is much too large as you have learned. I had to use cinder blocks in the opening of my bantam coop to keep goats from crawling through of at least sticking their heads into the run to eat whatever they can grab. We have spent decades trying to outsmart the goats here. When we think we finally won they find some other way through stuff.So yesterday I decided to goat proof the chicken coop. I made the opening into the coop 12”x12” along with a separating door with a 12”x12” opening. I didn’t want the goats eating the chicken feed. About a half hour later I am looking out the back door and noticed the goats in chicken pen. I said great they broke the door down. Walked down to check things out and noticed the door perfectly intact. Here come Daisy heading into the chicken coop!! What the heck. I never thought they would fit in that small of an opening. Now I changed both openings to 8”x8”. That has taken care of the problem. Darn goats are slick little critters.