I get the round bars and then have the Cub Scout drill a hole into the side of the car, putting the weights into the hole(s), then covering with wood putty to make it look better.  When drilling make sure the car is securely held in a vise, for safety.

 2.  Use GRAPHITE, lots of it.  Dip a pipe cleaner into graphite and run it through the wheel hub.  This really gets the graphite in there

 3.  Not all axles (actually it is a nail) are created equal.  Some of the nails have more burrs or are not as smooth on the underside of the nail head.  Remove the burrs with a fine tooth file.  Check by spinning the same wheel on different axles.

 4.  The center of gravity (front to back balance point) should be about 1-1/4 inches in front of the rear axle.  Some people recommend 1 inch.  Some people recommend cutting off some of the back end so that the rear axle is closer to the back of the car (however other people do not recommend shortening the car).

 5.  Not all wheels are created equal.  Spin each wheel on the same axle (actually it is a nail) to see which ones are the best.  You are not allowed to alter the wheels except to take off any burrs from the molding process.

 6.  Check wheel alignment by rolling the car on a smooth flat surface for a distance of 3 feet.  It should not veer off course.  If it does you may want to swap wheels from side to side or front to back.  Insert axles so that wheels are about 1/8 inch away from car, in other words do not insert axles so far in that wheel touches against side of car.

 7.  It is strongly recommended that you glue the axles in place with wood glue or epoxy after all adjustments are made.  But do not go over the 5 ounce weight limit.