Add me to the bite-the-bullet-and-build-what-will-last contingent.
My husband and I couldn't build a box either. We hired a carpenter. He was the biggest part of the expense to be sure but, still, we got what we wanted, what fit the space we had available and what has held up admirably for 3 years and will go for many many more. In addition, I live in an area that has just about all the predators (but bear, thank god!) including raccoon, coyotes, mountain lions, birds of prey and so on and haven't lost a single bird to a predator or even had an assault that I've been aware of.
Money well spent! ...if you want to have chickens and understand that they become your responsibility to keep safe.
The Carolina Coops are really beautiful and well designed. I think our coop ended up costing almost as much because of that expense of a professional and all new materials. Or maybe when you start adding up shipping and accessories we made our best choice. Who knows? And who cares? It's over. We get eggs. Our chickens are safe.
We used
these $10 plans and got an excellent result. Note that there's a free version and a very clear tutorial as well. And, if you have NextDoor or Craig's List, you might find people trying to rehome building materials and save a few bucks there. But use sturdy materials that will hold up to predators, the elements and time. I suspect, in the end, it won't be any more expensive and you'll have a better and a more stress-free time chicken keeping.
PS If you decide to investigate that tutorial and give it a go, keep in mind your chickens won't complain if things are out of square or don't look great. They just want to be protected. And power tools can be rented from your local building store. Maybe there's even a vocational school in your vicinity that would consider doing sections as a class project and a kid or kids who would be happy to finish it on site for a few bucks.