Sounds like a good plan to me. I made my daughter pillow shams to match her quilt and used muslin to back the tops of them, then had them quilted to match the quilt. So far it has worked very well. I did use batting, but I see no reason you would have to. Be sure to wash and dry the muslin a couple of times so it will shrink before you wash the finished duvet.

beverly

I don't think the top would survive washings long if it wasn't quilted down to something. I think I'd put a back to it in maybe flannel and quilt it like a normal quilt before I assembled the duvet. That would hold it together well. I'd also do french seams around edges to seal it for washing.

What is wrong with using a sheet to back it??If you arn't going to quilt it I feel that would be reasonable thing to do.
Walmart has inexpensive flat sheets at very reasonable prices and obviously withstand washings.

Thanks everyone for the tips!
Looks like I might need to do at least a bit of quilting, just to tack the quilt top to the backing so that it can withstand repeated machine washings. Good idea to use a sheet for the backing, as well as for the other side of the duvet cover. Then I don't have to worry about shrinking or wrinkling muslin.
Might be my project for NEXT winter!