Cast concrete or some other material, igloo style.
That’s what I live in right now. But then again I am in Afghanistan. Would not recommend it on a permanent basis.
I stayed in a “B-Hut” when on the Camp. The permanent (2006) party stayed in the “new” cargo city blocks. They had them stacked with walkways. Unfortunately, they managed to crush an electrical supply cable and short out the power to the entire structure. Before the power went out, one SNCO complained he was getting shocked by his bed. The Sergeant Major told him it was his imagination.
At least with the B-Hut, you had a compartment and privacy. Most of the people in the new digs had to share with one roommate.