I often make valances like this. I interline them with twill (you know, bottomweight wovens but lighter than denim) because it causes them to hang straight and it's OPAQUE. That is the main drawback to quilt curtains - the light shining through them shows all the seam allowances and washes out the actual pattern on the front.
You can see some here: even with that very bright sun behind them, you can still see the quilt pattern.
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This one isn't interlined yet.
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Here it is finished:
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The twill I used was a beige color, because that is what I had in my stash. You wouldn't want anything dark unless your quilt was dark.