Nice! I had one that was 4'x8' but I soon found it was a pain. Too big for a good lid, too big to easily catch babies without stepping into it, and a pain to clean out with sand for the bedding (which soon meant sand around the brooder from stepping i. I really wish it had been built out like yours, which looks like a 2'x8'. Much easier to reach across and to put a top on to prevent flying out.
If I did it again though I'd have at least one end where the side board slid into place. That way it could be removed to sweep out the bedding into a big broom catch-pan. I'd also elevate it off the floor so there is airflow under it. Random spillage from the waterer meant the wood would get soaked and stay wet; the sand didn't help matters but I suspect other bedding would have a similar problem unless it was super absorbent. I think the airflow underneath would have helped it dry out when that happened. Or perhaps just sealing the wood with that black spray people use on their coops would have worked, and a drain hole just in case into a Tupperware tub. Now you have me planning the rebuild!
Love your design, what are your plans for preventing flyers from escaping?