I'm exaggerating... but I would imagine I built a dozen of them.
I live in the flattest place on earth. Around here a two foot mound we call a hill... 5' is an unscalable mountain.
Behind my shop we have a loading ramp. Large chunk of concrete maybe 4' high. Used to load equipment on flatbeds. THAT was my soap box mountain. I had a stake, (carved myself, very ornate), that marked my 'gliding record'. I scrounged parts from every shop yard within a couple of miles of our shop. Red wagon tires and axles... bicycle tires... wood... anything I could use to build and improve my soapbox cars. I even built a steering column once... gear and all.... so no 'rope pull'.
For years I searched for an opponent to race down our loading ramp - then one day a kid about my age from down the street built one. The big day came... we pushed them up to the top of the ramp and another friend was the flag man. He dropped the flag and away we went.... but the other kid had no experience... went off the side of the ramp and into the used oil barrel... soaking him from head to toe. His mom never allowed him to race again... and so ended my soap box racing league. One race. I was the one and only winner in its history. - JamesJM