Whew! Finally done!
I decided to back the top afterall. I figured if I'm going to this much trouble to make a duvet cover, I didn't want to risk having any of the seams come apart. I'm quite happy with the result.
I found a 100% cotton sateen sheet (for $11!!) at Zellers to use for the back of the duvet cover. The top is machine quilted "in the ditch". I think I may add buttons to the open end to close it (safety pins are doing the job right now).
Surprisingly (ha) it matches my new "art"... :D