That sounds like a pretty good idea. I could just put a wood panel behind it. But the ground underneath has a small dip from when something tried to get under it. What can I do about that?For the door I would make it overlap so when you close it there is wood behind the door so when you close it the crack is covered on the backside, you could do this to the top and bottom, dont know if it would work well with that door though and it would probably be best to make a new one.Then make sure it closes tightly against the wood and then latch it. For the holes in between the rafters use hardware cloth or cut plywood out to screw in.
There aren't any holes lower down that would cause a draft. The closest thing to that is a small hole for the chickens to go into the outdoor part of the pen, which is fenced-in with very high fence. But that's closed off for now because I still need to check the fence for holes and see if I need to replace the large door for the outdoor chicken pen. So for now, I have a fiberglass panel and some cinder blocks covering it to that the chickens won't accidentally wander outside of the pain once they're old enough to leave the brooder.The door may need to re-hung that door to make it sound. Depending on the framework of the door, if its sound, a tad larger screw, or fill the old holes with a dowel rod, cut it flush with the jam, and put the screws back in. If the frame isnt sound, and has some rot, you will need some new lumber for that to re-hang it. That should take care of that gap. How far up are the soffits? If they are above the roosts, just cover the gaps with hardware cloth. But, if you have some that will cause a draft lower, fill those with 2x4s, and nail into place. Depending on the overhang of your roof, you may be just fine with the hardware cloth.