Have your Scout draw a design on paper then cut it out and use it as a template. Use the paper with the little squares on it to make it easier for them. Draw a side and top view on the paper by tracing around the block of wood.
Keep the car a full seven inches. It has to do with the physics of velocity and length of travel of the weights.
Use the full 2 3/4 inches (outside wheel to outside wheel) that the rules give you. This will allow the wheels to travel farther before hitting the center strip.
Leave a lot of wood in the back to put in the weights.
Use the groove closest to the end of the block of wood as the rear axle.
Use your imagination. Be creative. Shape has the least to do with winning. A beaver driving a log or even a pickup truck is more interesting than a wedge and will be just as fast. The aerodynamics of a small block of wood doesn't mean much in forty feet.
Use lots of graphite and spin the wheels on the axles. Once you match a wheel and axle together with graphite, keep them together. They wear into each other as a matched set.
Axles must be in straight front to back. That is square to the body. True the axles, don't trust the slots! If you have one, use a drill press to ensure all axles are straight. All four wheels must touch, no tripods.
After pressing in the axles, test the car for crooked wheels...roll it on the floor. If the wheels are on straight, the car should roll 8-10 feet in a fairly straight line. Should the car turn left or right, you need to tinker with the axle placement without removing them from the car body, until it rolls straight.
Glue the axles in place. Nothing is worse than having the wheel fall off as you cross the finish line.
Get the weight as close to the 5 ounce limit as possible. Add the last little bit of weight with lead tape from the golf shop. This can be trimmed with scissors at the last minute. Remember, the official scale may not weigh the same as yours.
Everyone has an opinion on where to put the weight. The majority believe that the weight needs to be predominantly in the rear so that gravity can act upon the weight further up the incline and for a longer period of time. A car with more weight to the rear generally grabs more speed down the slope. Many suggest having the center of gravity at 1 to 11/2 inches in front of the rear wheels. But be careful not to put too much in the rear or you'll pop a wheelie.
What kind of weight? The melted lead can be dangerous and unnecessary. Tubular weights can be sunk in the sides; flat weights, like those sold at hobby & council stores can be attached to the car bottom if it is carved in a bit. Incremental weights ( with pre-marked grooves) are easier to snap off into the size you need. Some just use BB's, nuts & bolts, etc., but these must be glued so that they can not move. No movable weights or mercury are allowed.
Keep the weight low on the car and in the center (Left/Right of the car). Put the weight just in front or behind the rear wheels for less wheel chatter.