Airflow was why I had to redesign I wrapped the frame in plastic to try and get stuff in the ground sooner, sort of worked but it was terrible as soon as it warmed up.Bulk rolls of landscape fabric are a favorite here. I have been lucky enough to find partial bolts of synthetic fabrics at second hand stores also in the past. Hope you have plenty of airflow in the house.
Didn't really think about actual fabrics. I was looking at something like this: https://www.gardeners.com/buy/summer-row-covers-garden-fabric/8609573.html?VariationId=P_5637148782
Edit: I did climb inside my plastic cave and closed the flap to hotbox it a bunch of times. Cracked me up.