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Generous of you to assume I can make it tight enough to trap moisture. If I did it, I'll probably cut some channels into the bottom section of framing.
@NickCJ yes probably, curtain like that will get filthy at the moment though, and I already have most the materials needed. The door will be replaced in spring once I have the money to do that. But just want something to reduce heat over winter.
You only really have a ledge and brace door for economy of materials and low cost of manufacture. Hacking that into an insulated door with more labour and materials is fine unless you introduce any traps for moisture.