Make a rectangle on the ground, 5 feet wide by 8 feet long, with chalk or a chalk snap. Using a cutting grinder, saw or cutting torch, cut the 2- by 2-inch steel tubing to fit the rectangle and weld the corners together. Repeat until you have three rectangles. Add a section of tubing across the width and in the center of each rectangle for strength.
Cut eight 2- by 2-inch steel tubes to 1 foot in length. Weld one tube to each corner of the rectangle at a 90-degree angle. Weld one tube (totaling four) onto the midpoint of each side, also at a 90-degree angle, to form the base of the trailer's box.
Lay the second rectangular frame onto the top of the 1-foot-high tubes and weld all points together. Weld the axle's springs to the underside of the frame at the center point of the frame.
Install lighting kit according to manufacturer's directions. Weld a triangle, using the 2-by-2 steel tubing in 2-foot increments, to the front of the frame connecting the trailer hitch.
Attach plywood to the exterior of the frame with self-tapping screws, leaving the top alone. Cut and fit aluminum to the exterior. Using the final steel rectangle, cover one side with plywood and then with aluminum--this is the lid of the pop-up. Weld hinges to the front of the frame--which is closer to the hitch--to attach the lid to the frame.
Open the lid to a 35- or 45-degree angle. Secure the lid with 2-by-2 tubing, like a hood rod, measuring 4 to 6 feet long. Weld a hinge to the inside of the frame so it can fold away flat when it's not in use. Cut a section of steel tubing to 1 foot in length and cut the tube in half down its bias. With the hood rod in place and the lid open, weld the cut tubing on both sides of the hood rod to form a sleeve. Make sure to weld it to the inside edge of the hood and not to the hood rod.
Fasten the screw-on snaps about every 4 inches along the three sides of the frame that will use the tarp for cover. Match the location of the snaps with the lid. Cut the tarp to fit (doubling over is a good idea). Make sure the tarp is in two pieces that meet where the front of the trailer is--near the tow hitch. Add the snap heads to the tarp and snap on the tarp. Add a mattress or foam pad to the trailer.
