A simple greenhouse design by Urban Farmer Curtis Stone that is fast and cheap to install. Also known as Caterpillar Tunnels. Cost is about $1,000 for a 12’ x 100’ poly tunnel made with 1-3/8” galvanized top rail tubing that’s used on chain link fences. Metal hoops are 4’ apart.
YouTube
I love, love low cost greenhouses. This looks like a good design (along with everything else Curtis Stone does).
Curtis Stone is currently building a larger version poly tunnel you might want to take a look at: https://www.youtube.com/watch?v=WxXU54KiHZI
Again, his videos are packed with useful details.