Is there a simple method of screeding the concrete on a 20-degree slope? This is a small job that doesn't warrant bringing in any special equipment.
A weighted, steel-face strikeoff should be made about 27 inches wide and whatever length is needed say 12 or 14 feet if the screeds will be 10 feet apart. When the concrete has been placed the weighted strikeoff is supported on the lower ends of the screeds and drawn up the slope. Concrete vibrators handled manually should be used in the concrete and moved up the slope just ahead of the strikeoff. The strikeoff itself should not be vibrated. Vibrating the strikeoff would make the concrete swell in height after the strikeoff passes over it.