The biggest problem with building an outdoor flight or aviary of any size is the metal components used to make the enclosure. Most of the hardware cloth or other wire, such as chain link that is normally available to the DIYer is not suited to use with birds due to the types of metal used as well as the coatings on them. The majority of these products are made of galvanized metals, which is to say that it contains a huge amount of zinc, which is highly toxic to birds.
ideally the way to go is with stainless steel wire mesh or even aluminum wire mesh, both of which are expensive, and usually difficult to find. I have found some all aluminum chain link fence mesh in 5o foot rolls, a 2 inch mesh X 120 inch high ( 10 ft.) runs about $921.00 plus shipping and you can usually find the hardware locally to put it all together. Stainless steel is even more expensive to get.
So far in chainlink I have not been able to find any smaller of a mesh and feel that a 2 inch mesh is too big for most of the birds that we have as all of my birds would be able to get their heads through this size of mesh, the largest bird that I have is an Amazon.
Despite the cost of the materials, I am still looking and hope to be able to build an aviary for my birds sometime this year. I am glad that you made this post as I have been lax in looking for materials this winter and need to make this a higher priority now that spring is in the air. Allow me to have a couple more days and I will post an up to date list of links to materials for this project.