DUDE!!!! i did the cardboard thing also
...but it was mainly for my room so i could race in the middle of the night...or whtever
yup i found these big perfect boxes that didnt have folds in them..and had two...3x2' foot of flat space....it was perfect
i connected 2 of them with tape.....and then put tapes on the folds so that my car could make it over them easily
so in general..i have like 10 feet of space on the cardboard to race in my room ....
normally i would race in my backyard or in the driveway