Use a layout jig & chainsaw to make a rectangle 9" x 9" x 20"

Multiply the width of the cylinder by .29 (in this case 9 x .29=2.61)

2.5 is then what you should set your layout jig for

Scribe 8 lines on the rectangle (2 on each of the four 20" sides)

Using a chainsaw, cut off the corners. 

Really focus on your cuts, making sure to not cut into the cylinder & not stray away from the line.

Notice the foam under the carving?  This protects your saw

Using a Nail sander, Gently knock off the 8 corners created by your chainsaw

Using a Small scratchy board Scratch the surface until round

Using a pistol grip sander, sand the whole surface

Finished cylinder after spraying with a hose

Finished Cylinders incorporated into a designs