While I like the idea of a curtain rod shelf, I have done something similar but with what I'll call a valence.
I have made wood enclosures which can fit over the tops of the widow hiding the top of the curtain. I hang them using a french cleat, so leveling is no problem.
1. Hang the curtain rods from inside of the enclosure.
2. Use any design to match the room or decor. I often use custom molding; router made.
3. The top can be used as shelf or left open.
4. I frequently use a three corner match with mitered corners.
5. Inlays can be used. I have used cartoon types for kids rooms and wood veneer inlays for some high end finishes. Inlay borders.
6. Stones, pictures, endless items can be inlayed into the enclosures.
7. You can paint or stain.
(In one case, I even made a recessed compartment in top so a rifle could be stored above. Someone wanted for their bedroom.)
Just some things I have done with idea similar to yours.
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