Cheap storm shelter designs include an above-ground safe room made from plywood and steel or an underground concrete structure using a modified septic tank. With both these designs, the owner must do some of the work himself to keep the budget affordable.
Construct the safe room by bolting the frame to the concrete slab floor. Screw a double layer of 3/4-inch plywood and 14-gauge steel on all walls and the ceiling. Orient the sheets of plywood so the grain runs at perpendicular angles, and add the steel layer on the inside of both sheets of plywood. Once the safe room construction is complete, finish the exterior of the room with drywall for seamless integration into the remainder of the home. A steel door gives easy access to the room.
The septic tank shelter is an underground concrete structure. It requires working with a manufacturer that is willing to make modifications to reinforce the tank and add air vents. Reduce the installation cost by digging into a hillside. Cut an opening for a door using a concrete saw, and add a door that is able to withstand a storm. Before building the shelter, select a site that is easily accessible from the home.