I got my kits from Greggs bird houses
There's a company called Timber Pro Coatings out of Oregon I think. They sell a product that people put on their raised bed gardens. If a guy was going to use untreated lumber it might work. My experience with OSB plywood hasn't been that good. Biggest problem is the squirrels chewing it up.
Must be this product?: https://timberprocoatingsusa.com/products/internal-wood-stabilizer/
The biggest problem I've had with my pine ones is the roofs cracking and/or warping. The one-piece ones. The deck stain does help.