Glue this to the inside, seal the edges with silicone. Mine has lasted 40 years with just two 2" holes on top for vents.
Coroplast 48 in. x 96 in. x 0.157 in. White Corrugated Plastic Sheet-CP4896S - The Home Depot
Coroplast 48 in. x 96 in. x 0.157 in. White Corrugated Plastic Sheet-CP4896S - The Home Depot