I'd add :to the above tips- weight- to the rear and as high as reasonably possible. The track start is elevated. The higher the weight, the greater the potential energy= greater terminal speed, all other things being equal.
I helped my son make a rail, similar to the gold one pictured, with a lead rear spoiler. met all specs and won easily. Just be careful it doesn't tend to wheelie.
Also, if you don't have a lathe, a drill press or even a hand drill (properly secured) will do fine to hold the nails for polishing.