It looks like several pieces of trim were used to build the cornice, from crown molding to quarter round to a 1x4. Nice work on the miter board. Then the fabric can be attached any way you like--velcro would be easy to maintain. If you click on "more info" below the picture, you'll see a link to Chris' blog. Bloggers often give details, so check it out!

I think u can even make a use of window box to make these canopy(the window planters) they sell it lowes and homedepot) wht u can do to make look like this canopy is turn the window box upside down and paint it with anycolor of ur choice and inside of the window box u can add maybe some hooks or even put small railing for curtain since it would be all inside the box u wont be able to see. I know its crazy idea but hey its cheap and u can have coustom made canopy ( one thing if u could make sure find thoes window box for this project which doesnt have soo much hole on it .. Enjoy.

Amanda's room window