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yikes.... you would need to build a small frame to compensate for the difference and make the rough opening the 21 inches. But that would mean that you would need wood 5/8" thick. You could attempt to use 1x lumber, which is 3/4" thick.
What is the window currently affixed to? (is there existing framing wood that you could attach to along the top and side?) If so, you could just use 2x lumber along on side and along the bottom and shim the window so that it is square, plumb and level...
Good luck...
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