I used to make great use of some galv steel in 6x3 sheets, that we used to call "chicken tin" Thin steel, probably .5mm , galv both sides and a light green etch primer on 1 side.used for rat proofing among many other things. you could cut it really easy and nail straight through it. You could do with a bit across the bottom of that door, if you are putting chicken feed in there! I wanted some a few weeks ago, and went to the place I have bought it many years ago, to be told "sorry mate, never eard of it" Thats progress for you!
Phil