Step 1: Materials
(2) 2x4 4 feet in length (I cut an 8 footer in half)
(4) 2x4 6 inches in length
(4) 2x2 8 feet in length (if you are carrying more rigid materials you can use 3)
2 1/2" Deck screws
Foam Padding, or carpeting - closed cell foam sleeping bag pads work great
Shims - but any thin scrap wood will do
Staple Gun (tacks and hammer will do)
Step 2: Cut and Wrap the Feet
These will be the feet that rest on the roof of your car. To prevent them from scratching the paint, and to provide a little more friction than bare wood. We'll wrap them in some padding. I used some closed cell foam sleepingbag pad.
Cut four pieces of padding to about 6 1/2 by 6 1/2 inches
Using a staple gun or tacks (roofing nails have nice big heads too) fasten the padding one the EDGE of the 2x4's so that your staples/tacks/nails to not come into contact with the roof of the car.
Step 3: Attach the Cross Members
Lay one of the 2x4's across both feet and try to center it as best you can. You will notice that due the curve of the roof, the 2x4 crossmember does not lay flat on the feet. This is ok.
Place a shim (a wedge shaped piece of wood) or any thin scrap wood in the gap between the foot and the cross member and drive one screw through the cross member nearest the point where the foot and cross member touch (but at least 1/2 inch from the edge of the wood). Drive a second screw in the opposite corner of the same joint, through the shim/scrapwood.
Don't worry when the shim/scrapwood breaks and falls out, we only needed it to provide a spacer. Once the screw is on place it has performed it's function.
Repeat for the remaining three feet.
Step 4: Add the Rails
Try to allign your cross members and rails so that they are square. You can use your car for reference.
Use a shim or scrap wood to fill the gap while you drive two screws from each rail into each cross member.
Step 5: Get a Rope
While driving, PLEASE keep an eye on your rack and the materials you are transporting. The wind forces that can get under your drywall are proportional to your driving speed (and wind conditions in general). Take back roads, do not drive on the highway, be aware and be careful.