Use the cult for info. We free handed ours and it turned out pretty well.
Our longest feature is a kink rail which marks a little over 10ft.
I wish we could make a 20ft box only because 10ft doesn't give you much space to do anything, but you'll have to make a decent drop in to gain enough speed to clear a 20 footer.
FYI, PVC pipe is sold in 8' increments at most hardware stores. You'll have to combine multiple pipes to make a longer rail.