Measure the outside of your calender, don't add any seam allowance at this point. Measure the complete width backside, spine and frontside - a tape measure works best for this.
If you want to make your cover softer cut a piece of batting in the same size.
Fold each of these 2 cut pieces into half, right side out and iron them. These are your 2 flaps.
Turn the right side of the cover out through the opening, iron and close the opening with hand stitches using the leftover thread.
I'm linking this post to Nina-Marie's "Off the Wall Fridays" blog.