
Balance the car on a two-ounce plastic portion container (such as the Solo Soufflés brand cups) turned upside-down, and then insert a push-pin or thumbtack from the inside up into the bottom of the car. Done.

You know... the ones that are about 1-1/4" tall and 2-1/2" in diameter... also known as "sauce" cups. We get these 2 oz. cups regularly filled with condiments at restaurants, or with free sample-sized servings at the local wholesale club. I prefer the black or translucent cups over the clear ones, because they won't show the fastener underneath (the picture uses a clear one to visualize the push pin). Most varieties I've come across are dishwasher safe - just wash and then recycle for derby purposes.
Why do this?
1) The cup tacked on the bottom elevates the wheels, so it won't roll away, and it is impossible for playful hands to roll it around.
2) The cup is securely fastened, so the display base won't separate whenever the car is admirably picked up.
3) The inverted plastic cup is tapered, so it is stable and difficult to tip over.
4) The 2-oz. cup is low-profile, so if it is tipped over somehow, it won't fall very far.
5) It is durable, yet lightweight, cheap, and disposable.
6) It is easily removed, and then only leaves an invisible pin-size hole underneath.
One downside is that repeated removal can enlarge the original pin hole and make it looser, requiring the hole to be relocated.
Other portion-cup sizes also work, but the bottoms of the two-ounce varieties are known to be approximately the same width as the pinewood derby block (1-3/4").
