Extending rotten mullion

I replaced a rotten brick molding and rotten window sill nose on the exterior of my dining room window. The mullion had some rot on the end of it where it intersected the old sill nose that I removed. I couldn't find another mullion at the home improvement store that matched the one that I have on my window so I have cut the rot away from the end of the mullion plan to use wood filler to extend the end of the mullion to intersect the sill nose.


Do you think this will work or have you a better idea

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