The size of the box can vary according to need. Mine is roughly 2 ft. I used two ten foot 2 x 10 and one ten foot 2 x 4. You could use 2 x 6's or 2 x 8's and make 3 sections to construct a deeper box. Heavy plastic could be stapled to the top frame instead to cut costs.
Dimensions can vary so none are given here.
The top slopes about six inches. This makes the bevel on the front and rear boards of the top section approximately 15 degrees. The prop should be long enough to fit easily inside the side piece of the top frame. The large wood screw is used to fasten the prop to the front inside face of the side piece. Hold the prop against the top frame and drill a hole through it and into the frame.