im gonna do that too, as soon as this rain stops....but i was thinking like a frame of a box made of pipes, maybe 10 ft tall and like 10 feet across for lacheys, maybe a smaller version of that for learning new things then take it to the big one? also scaffolding are good.
check out tempest freerunning academy on youtube, they have a lot of things that would be easy to build, good luck man