My Dad and I made ours together in the 1980’s, but I did the vast majority of the work. He helped and, well, told me what to do. We started with the basic block most years and occasionally bought a prefab one, which was not painted or even completely formed. It just had a bit of “car” shape to it and we did the rest. And it did not come with decals; we bought our own(that was a lot of fun).
You could always tell which kids had Dads who had really done most of the work.
I won our local one twice, but always lost regionals because they banned graphite lubrication due to the fact that it stained their floors.