Thanks for your insight -- esp. about the part where sand can get through even the top vents!
If I were to rip out all the mesh and replace with solid fabric -- and punch out a few zippable mesh-backed windows/vents -- can you tell me what's the thinnest/lightest fabric I can use that's still reasonably durable? The material need not be superstrong since the fly will block wind and rain -- but I suppose it shouldn't be tissue-paper fragile either. Your thoughts?