This instructable will show you how to create a "pocket" underneath your son's deby car for easy weight addition/removal.
Get the closest possible to the maximum5.0 oz.

Step 1: Acquire All the Parts

Materials:
1 Ea. Pinewood Derby boxed set.
Contents:
Wooden block
4 Plastic wheels
4 Metal "axles" (nails)
Set of fishing weights
1 Ea. 35mm Film Canister with lid
Fast Setting Epoxy or Hot Melt Glue

Tools:
Drill Press
2 Forstener bits (1 the size of the cannister, and the other slightly larger than the lid)
Dremel Tool with sanding drum attachment
(You can forgo the drill press and bits, but you won't get the tight fit that looks good.)

Step 2: Drill a Hole in the Car Body

Find a position to drill on the car body as close to the back end as possible.

1) Flip the car over on its back.
2) Using the drill press and the larger Forstener bit, Drill a hole slightly deeper than the thickness of the film canister cap.
3) Replace the bit with the smaller bit and drill a second hole, this time drill as deep into the car body as you can without protruding on the other side.
4) With the Dremel tool, sand a notch on one side of the canister so you can get your finger under the cap to remove it.
5) Test fit the canister and lid to the hole.

Step 3: Glue the Canister to the Car Body

1) Cut the canister in half.
2) Add some glue to the inside wall of the smaller hole that you just drilled in the car body. Use just enough glue to attach the canister to the wood. You don't want it to dribble all over the place, specially between the cap and the canister.
3) With the cap on the canister, slowly insert the canister into the car body until the top of the canister lid is flush with the bottom of the car.
4) Turn the car over and sit it on a flat surface.
5) Put a heavy weight on the car so the glue dries with the lid sitting flush.

Step 4: Weigh the Car and Add Lead Weights As Needed

1) Using a popsicle stick, or screwdriver, carefully pry the lid off the now securely attached canister.
2) Flip the car upside down on a postage scale, and weigh the complete car, with the lid on the scale as well.
3) Add weights into the canister until the scale shows the regulation 5oz.
4) Close the lid tightly.

Step 5: Gentlemen, Start Your Engines.

You are ready to race.

During the day of the race cars, and owners, will go through registration where the cars will be weighed. This is where this project pays for itself. Invariably, your scale, and the judges scale, will show different weights due to manufacturer, curvature of the earth, distance from your home to the Earth's core vs distance of the race location to the Earth's core, relative humidity, low battery, high tide, low tide, Sun spots, Van Allen Belts, etc., etc. Simply pop the lid off, and add or subtract weights until 5oz is achieved once more.

"WYLIE COYOTE - SUUUUPAAAAA GENIUS!!!"