Thermarest Z-Lite sleeping pad folded in half. It is decidedly firm foam, and 2 layers of it provide enough insulation for sleeping on snow. If the silver look bothers you; throw that REI fleece blanket over it.

If you want to be fancy, here is an expensive solution that is just plain hard: https://www.amazon.com/Oriental-Furniture-Half-Size-Tatami/dp/B000KDM5ZK/ref=mp_s_a_1_9?dchild=1&keywords=tatami+mat&qid=1610084826&sprefix=tatami+&sr=8-9