1/4" hardboard (Masonite) template for the ends. I traced the lines on the board from the template, cut close to the lines on the bandsaw, then used a template bit in the router to trim to final dimensions.
Once the outside was cut to dimension, I used another template to route a 1/4" wide by 5/16" deep groove tambour track. I used a solid carbide 1/4" upcut bit in the router with a pattern collar.
Flip the pattern to the other side for the other inside cut.
Mr. Hays' design used a smooth/continuous arc for the tambour track. I varied from that design because I wanted to make a bit more room available on the inside.