What an awesome opportunity to be creative. Definitely use hardware cloth or welded wire, especially on the bottom portion (at least the bottom two feet, preferably more if possible) to keep predators out. Personally, I would tend to locate the laying boxes towards a corner - you could easily make a coop in a corner to keep the birds more protected at night and place the nesting boxes to the outside of the existing structure for ease of collection. This would also give the chickens a place to get out of the wind should you ever have a "cold" snap. You could build the coop so that it utilizes one of the half doors and you have easy access to the inside for ease of cleaning. The only other thing that comes to mind is, since you have plenty of height in this structure, you could use part of a tree (some trunk plus some branches) as a roosting option/adventure area for the chickens. Good luck and please share pictures as you begin to build.
