Standard aviary builds would include at least one wall normally at the rear. This serves as a place to mount nest boxes, etc but more importantly it stops animals (owls, hawks) chasing the birds from one side of the cage to the other.
Due to high winds a mesh window between the roof area and the rear wall could be used to reduce the air pressure.
If you are using 2x4 timber the max unsupported span really wants to be reduced to 12'
these are cookie cutter aviary's I have built they basically follow the same design. One is 8 wide and the other is 12 wide. (3rd is a repeat of the 12')
The rear window (meshed) is to reduce wind load on the rear wall, The cross brace in the front is to reduce twist and to support the length of the front bar with out having to make the bar thicker (thicker would work but then it becomes more visible). Each corner has a stake driven in the ground then screwed to the base as we also suffer from very high winds.
The base frame is lumber that I expect to have to replace one day due to the wet ground during summer so its not used as part of the frame. The 8 foot used landscaping timbers where the 12 was easier to use 2x4. (landscaping timber looks better)
The side wall are about 20' wide which hides the extra support before the door. I don't use nails so everything is screwed with torq head decking screws so its solid as a rock (we have towed one recently with a pick up truck to move it a few feet further apart so I know its over built) also the torq heads don't strip and are easy to remove even after a few years.
The sheets you see laying on the ground are made to hang on hooks at the top of the aviary. There's a steal pipe in the bottom which keeps the bottom of the sheet against the aviary then the sides attach to hooks covering all of the font and side mesh. The rear window has a separate piece that's also attached by hooks.
Night lights are 12v trailer side markers in amber that come on automatically. These were installed after a bird flew in to a perch at night and broke its neck. The lights are dim enough that the birds stay quite but bright enough to allow them to fly if they get spooked
Im not saying this is a good design im just trying to point out some of the things that I have learnt from other people who have aviary's and what you should be thinking about during your design.
late afternoon when its getting cold and dark is not the best time to be working out how to cover a aviary so try to plan the cover into your original design.
Looking forward to seeing you designs and construction method
p.s any good ideas you have will be copied .....