i am just getting ready to try my first pinewood derby car, What I did to create the prat file was start by drawing a rectangle that matched the block size and also created rectangle to match the precut grooves for the axels.(In some places you are free to move the axel locations where ever you want but in our district you may not) Then I created lines and curves to build the shape of my car. As long as everything stays inside the original rectangle and you are mindful to retain you wheel spacing your good to go.
"The most valuable tool I have is the experience of others" - Me