Cut two 24-inch pieces off the length of square tubing. Then cut off two 12-inch pieces. There should be one 48-inch piece of square tubing left.
Create the two end T-shapes on the floor. Before you weld each T-shape, connect the welder grounding cord to the metal tube. Weld the two T-shapes. Stand the T-shapes up 48 inches apart. Set the 48-inch piece of square tubing on top of the upside-down T-shapes and weld the pieces together.
Clamp the 48-inch piece of strap iron on top of the frame you just welded. Measure and mark holes in the center of the strap 4 inches in from both ends and 24 inches in (i.e., the center). Drill the three holes with a 1/2 drill bit.
Unclamp the 48-inch piece of strap iron from the top of the frame. Clamp the newly drilled strap to the center of the 4 foot of 10-by-1/2 inch piece of plywood lengthwise. Drill through the pre-drilled holes in the strap iron to create the three holes with a 1/2 drill bit through the piece of plywood.
Hammer the three carriage bolts through the holes until the top carriage part is sunk flushly into the piece of plywood.
Lay the piece of foam on the top side of the plywood. Lay the piece of fabric over the foam and staple it to the base of the plywood.
Place the strap iron over the bolts on the bottom of the plywood and then lower the bolts through the holes on the square tube frame. Place a lock washer on each bolt sticking through the bottom of the frame and then screw the nuts tight.