Matching the weights is probably the most important and I agree with "Mad" a bigger seam. I am in the process of creating a doona cover with quilted elements (continental quilt) and I am using a heap of "found" fabric...some of it offcuts from a curtain store (not backed). I always prewash to see how any unusual or non-traditional fabrics come up. Saves drama later. Look forward to seeing your work...and keep in mind, it's supposed to be fun and be bold, you can step outside the square (something I have to keep reminding myself!).