Low-cost, semi-permanent rigid-wall modular longhouse-type, peaked-roof structures, stadium-shaped (in plan view), made of light-weight, water-proof cellular extruded plastic sheet material die cut into blanks that are easily portable, quickly field erectable and dismountable by unskilled workers for a wide variety of uses. Two semi-circular end portions are spaced apart and joined by multiple segments of straight wall and roof modules, the length depending only on the number of intermediate segments. Blanks are easily folded on site into double-walled, bolt-together, modular wall and roof panels. Only four basic panel configurations are needed to form all structural elements. Multiple modular structures may be grouped to form larger compound structures with individual ones assigned to individual persons or dedicated work space usages: disaster shelters, sleeping, gathering, storage, hygiene, medical treatment, schooling, entertainment, goods manufacture, cooking, eating, herding, etc. Double-wall modules provide inter-stitial space for cold-weather insulation and passive ventilation in hot environments.