In our case, the metal roof was hand built by my father-in-law for his granddaughter out of some aluminum sheet roofing that was literally lying around (our property came with a lot of spare parts), leftover fencing, and some metal pipe and some metal conduit. It's a pretty simple design. I'd be happy to take more pictures for anyone needing a better look at construction details.
If you just do a simple shed roof rather than a peaked roof, it's even simpler to build. There are many suppliers who make simple metal roof structures or kits meant for use in horse or other livestock shelters.