Good start and good idea to build a mock-up. I wondered why a rollout nest box instead of a full time bumpout? Easier to build a permanent bumpout box, and makes for more floor space inside. And be sure to locate it lower so that roost can be higher without putting chooks flat up against the hot roof decking. Looks like roost will be on tallest wall. Did you plan to build permanent vents at nest wall and opposite wall at the top? Good natural place to put them between each rafter. If you have the space, do some pre-fabbing whenever possible. Could build door, nest box, poop boards, etc now. Might be almost as easy to build it on a footprint of 8X6 as opposed to 8 x 4? How far does it have to be moved to site? Is it to be raised? If so, then make it 18" to 24" tall so no rodents can take up residence.
Take long breaks, short puffs, laugh a lot, and work safely.
Gerry