I bought a cheap green 6x3 polytunnel, 7 year's on it's still standing. Year 2 the zip on the door broke and the mesh on the Windows disintegrated, it then blew the cover off that winter despite being dug in. My husband decided to build a wooden frame 2 decking boards high with the posts in concrete. He put another decking board on top for an edge then nut & bolted the poly frame to the decking boards, we rolled the bottom of the cover around long wooden poles and held it all in place with long screws. Since then it's never moved and the extra height inside is a bonus, next time I want 3 decking boards high!
It's still bright inside for growing and the plants don't get scorched either.