:goodpost: I agree with her thoughts.
I have used 2x2s with 1/4 plywood to make 1/2 sized doors and it works fine for that, but I have not done it for a full sized door.
I primarily use deck screws for all of my building that way I can disassemble if necessary. I vary the length of screw based upon what I am connecting. With 2x2s it is important to drill a pilot hole before using a deck screw or you can split the board. I also would use deck screws with OSB.
Immunity is conveyed by a combination of age and exposure. The idea is to protect them right after hatch when coccidia is most dangerous to them. Assuming they are in the environment they will be living in, 6 to 8 weeks of medicated feed is plenty and you should begin weaning them off of it...