Drag spoilers are fairly simple devices that perform a complicated task. A well-designed spoiler will increase high-speed stability by slightly increasing air pressure over the trunk lid. That high pressure air pushes down on the back of the car, helping to keep it pointed straight at over 100 MPH. Drag spoilers are tremendously effective and fairly simple to build if you've got some fabrication experience.
Mark and cut two right-angle triangles measuring 24 by 8 by 25.3 inches from a piece of 6-gauge sheet metal. The 24-inch side will be the top of your spoiler supports, the 25.3-inch side is the bottom, and the 8-inch side is the front.
Measure 2 inches down from the top of the supports' front (8-inch) ends and make a reference mark. Draw a line from that reference mark to the back tip of the spoiler supports. This line represents the horizontal surface of your spoiler.
Hold one of the side supports up to the back of your trunk lid so that the reference mark lines up directly with the down-turn on your trunk lid and the top (24-inch) part is parallel to the ground. Begin marking, trimming and re-fitting the front of the triangle to the back of your trunk lid until it fits smoothly from the top of the trunk lid down to the rear of the car. The front of the triangle should end up with either a triangular or concave cut-out. Transfer this pattern to your other support.
Hold one of the supports against the trunk lid and have an assistant hold the other one where its respective position. Measure the distance between them and cut three pieces of steel tubing to that length.
Weld the end of one of your tubes to a vertical support exactly 1/4-inch from the cut-out on the front of your supports. The top side of the tube should fall exactly onto the reference line that you marked earlier. Weld the other end of the tube to the other support in an identical manner.
Weld the other cross-support as close to the rear tips of the triangular supports as possible, positioning it so that the top edge falls on your reference line. You'll wind up with two side supports connected by a front and rear cross-member.
Cut a piece of 10-gauge sheet metal large enough to fill the width between the side supports. Cut it long enough that it extends 1/2-inch past the rear cross-member and 3 inches past the front cross-member. Bend a 1/2-inch, 90-degree flange onto the back of the sheet metal. Place the sheet metal onto your spoiler frame and drill 1/8-inch holes through the sheet metal skin and cross-member. Use a pop-rivet gun and rivets to attach the skin to the frame.
Repeat the skin attachment procedure for the top of the front cross-member, leaving a 3-inch flap of sheet metal protruding past the front cross-member. Fit the front edges of your supports with weatherstripping then have an assistant hold the spoiler in place while you drill rivet holes through the 3-inch flap and the trunk lid. Continue riveting at 1-inch intervals until the spoiler is secure.
Weld your third cross-member to the inside-bottom of your vertical supports as close to the rear of the car as possible. This cross-member will both keep the vertical supports stable and reinforce the assembly under high pressure. You may need to use another pair of vertical supports if this horizontal member blocks your license plate or parachute.
Trim away the excess sheet metal from the back of your vertical supports with a grinder and trim away any excess rubber weatherstripping with a razor knife.
