A gradual incline would make all the difference for the outdoor theater.
Stadiums do it every year for motocross and auto rallies. They lay down blankets and boards to protect their pristine grass fields before piling dirt on top.
The pitch wouldn’t be steep at all. A rise of 20 feet over the course of a couple hundred yards would create a pleasant amphitheater environment. The back of the mound would drop off in a safe rounded pitch from its 20 foot peak in a 90 degree semi-circle stretching from the Gobi Food Court over to the Doolab and then across to the Beer Gardens. It would also provide a fantastic view back towards the rest of the festival.
The temporary elevation would be covered in Synthetic Turf, similar to ant-hills featured on playgrounds.
And there you go.
Feel free to get a sponsor for the damn thing to pay for it.