trevor's got it right. it can be done with flat, it's just not as easy and you find yourself always lacking speed
i went from this, needing soap and water (this is on an incline, hard to tell)
to this, needing less soap and water. (pretty flat)
to this, needing just water.
i have plastic chicken fencing on the initial drop in, and once the tranny starts, it's all turf and it works perfectly on a slight slope like in the third picture. it's super convenient to not need soap on the setup.