Proper wood

Elm would be good with it's intertwined grain structure.
If the weight of the window treatments suggest to you that splitting may be a problem... Assuming an elongated oval, ignore the grain orientation of the sawyer's work (i.e., grain orientation relative to sawn edges) and rotate a pattern of your oval (either clear or the negative of what you want, e.g., the hole left from cutting out the bracket) over the board so that the grain follows the lower half of the curve as close as you can. The top of the curve would then show end grain and the bottom would show long grain. This in effect angles the grain so it acts like an angled bracket.
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