Cut your plywood 12 inches wide, 12 inches tall on the sides and 15 inches tall at the peak.
Make a mark for the peak to the edge of the sides and line your saw blade up with that mark and cut. Should be about 41 degrees.
Cut the 1X2X8 down to match the cuts on the plywood.
Paint or stain your plywood and stain the 1X2X8's the color of your choice.
I chose Dark Walnut stain and painted two calendars white and green.