I did something similar to what you are suggesting but I did it by adding a purchased bed ruffle to the sides of my quilt. I was lucky to find one at the thrift store, brand new and matching the type fabrics I used in the quilt, which was a stack and whack pattern. I will try to explain what I did, lol.
First, I did the sewing birthing style, with only a sheet for backing as I wanted mine lightweight. OK, so: I layered it thusly; if you want batting, spread it flat as the bottom layer. Next lay the pieced top out flat, right side up. Next place your sidepanels facing inside the backing; line up the seam edges with the top edges. Now lay your backing, right side down on top of the panels. And pin, pin, pin the edges together. Stitch around the edges being sure to catch all layers and leaving an opening at the bottom for turning your layers right side out. When you turn it, the seam will be on the inside with the batting and the backing, top and panels will be on the outside. You will lose the width of the seam from the panels, but it shouldn't be noticable.
Mine was still a little too short, so I added a wide lace to the bottom of the ruffle. It's not perfect, but I was pretty happy with the results. Oh yes, I forgot to add the ruffle came with two pillow shams!
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