Not sure if you have used that cold frame before but just in case you havent… it heats up and cools down very quickly I put a 60% shade cloth over mine that is that size ( same one actually).
I have another one I made that is about 4x larger and even that one requires a shade cloth to slow the temp swings. This one is actually dug into the ground about 12inches but figured I would show the use of shade cloth
